A Noachide is a non-Jew who follows the sheva mitzvot B'Nei Noach. Being a Noachide is also called being one of the B'Nei Noach.
The main problem is that almost no rabbis have sat down and figured out what being a Noachide actually requires in real life situations. There are a few rabbis now trying to figure this out, but Judaism has thousands of years of rabbinic rulings and interpretation to figure out already-detailed instructions (the Torah). What can a non-Jew unlearned in Torah do with seven sentences? For that matter, there's little to no precedent for today's rabbis to use. Halachic rulings are based on precedent that stretches back to Moshe Rabbeinu himself, who was able to ask Hashem himself for clarification on the text. The seven Noachide laws skipped all that tradition. I honestly can't say I would feel comfortable trusting a ruling with such a lack of history. That situation is ripe for abuse, extremism, or just plain wrongness. (And my guess would be that it's not the moderate rabbis who feel a need to increase Noachide observance in the world...)
To my knowledge, the Noachide faith is the only "religion" that requires another "religion" to tell them what to do and how to do it. Except Judaism isn't doing that.
But there are other issues that can make a person decide to "skip" a "Noachide stage" and go directly to seeking conversion. Let's discuss them.
Lack of religious leadership. As stated above, the rabbis have historically had no interest in the Noachide laws, other than casually mentioning that they exist or deciding whether another religion can qualify as satisfying the laws. Today, there is no group to certify or train Noachide leaders. That can be dangerous, since anyone can set up shop and start recruiting. A cursory review of some of the Noachide "literature" on the internet reveals that there are some sketchy people out in the world. When you have a question, who do you ask? You could ask the local rabbi, but I'll bet money your local rabbi will be clueless on many of your questions. But the bigger problem is that there isn't even a "bigger" rabbi for him to call to ask. There's just not any rabbis doing this work (minus one in Israel, to my knowledge).
Who is your community? There are very few "Noachides" in the world. There are a few developed "communities," like less than five in the world. And you thought it was bad enough that your conversion required you to move within walking distance of an orthodox shul? Of course, a Noachide is not required to live in a Noachide community. In fact, you may not even like the people you find there. (There tends to be a lot of New Age and "off the grid" people.) Who will you call on when you suffer a death in the family? Want to celebrate a birth? Get married?? But is the Jewish community really your community?
Lack of houses of worship. Where do you worship as a Noachide? The "communities" have houses of worship, but that means there's not many options. Note that some religions qualify as satisfying the Noachide laws, but most people actively choosing to be a "Noachide" today are leaving those religions. For example, if you were a Muslim, you could continue to worship at the mosque and satisfy the 7 laws, but if you believed the Jewish religion is true, you could not honestly participate in the services. Worshipping at the community synagogue can get complicated, and men could accidentally be counted in a minyan. In the beginning, people might accidentally ask them to cook (and if they don't know the kosher laws, they might show up with their food!). There are a lot of things that can become embarrassing or complicated. You think it's annoying explaining to strangers that you're not Jewish but converting? Try explaining that you're not Jewish and not converting but are still davening in the shul!
What do you daven? You can't say most of the liturgy when you can't talk to Hashem about "our" ancestors. You can't thank Hashem for not making you born a gentile. Most of the liturgy is written in the plural and is spoken in the name of the Jewish people. There is a lot that just doesn't apply to you. Your average community rabbi likely isn't going to be able to answer these questions for you.
Who do you marry? Do you marry an atheist or agnostic? Do you marry a Christian or Muslim? How would you find another person affiliating as a Noachide? Would an atheist, Christian, or Muslim even be comfortable marrying you? I don't think Eharmony has provisions for Noachides. Worse, who would officiate? The county judge??
Assuming you marry (or were already married), how do you educate your children? There's no Sunday School for Noachides. Besides you, who will be their adult role models of being a good Noachide? Will their friends reflect your values? And in the future, who do they marry?
There are some people who, knowing all this, actively choose to be a Noachide in their community. I find this is generally people who, for whatever reason, cannot convert. Some are temporary Noachides, as they "try it out" to see if they want to pursue conversion or if life circumstances temporarily prevent them from learning about Judaism. This seems most common with people living in rural areas.
I think Noachidism is probably most common among people interested in converting but who are married to a non-Jew who is not interested in converting. (Remember, it's possible for a couple to both be converting.) My understanding is that only the Reform movement will perform a conversion in this case. (Maybe the reconstructionists do too, but the conservative movement does not.) For people in a loving relationship with another non-Jew, Noachidism is a good compromise, but the worry is that the Jewishly-inclined partner will view it as a compromise and develop resentment. Rightfully so, the rabbis do not encourage divorce in these cases. However, over the long-term, divorce may happen anyway.
So, my born-Jewish friends, don't be so surprised the next time a conversion candidate says they never seriously considered being a Noachide. One day, there will be an organized, fully-informed Noachide community, but that's when we will know Moshiach has come. For me, and for many others, the Noachide movement today is not developed enough for our spiritual and community needs. But one perspective is that dissatisfaction is a Jewish neshama saying, "This is not where you belong!"