The process as it relates to a K1 Visa is a multi-step series of approval gates designed to state that someone is “admissible” based on certain conditions, which change as you move through the process.

The general logic has been that it’s really easy for people to say they want to marry a U.S. citizen, get approved to emigrate, and then change their mind after (the common term for this is visa fraud). So the government grants a series of visas for increasing lengths as you move through that process and prove that it is a bona-fide relationship.

A K1 visa is the last step before getting married, and stipulates that you get married within a short time after entering the country, after which you have to remain married for several years, prove you’re doing things normal married couples do (like live together), and then you can get your permanent residency.

So, in short, it’s not as clear cut as a one-time yes/no decision. You very much live within a prescribed framework for several years until the government is satisfied that your relationship is real.

(Source: personal experience)

If they were here on a non-immigrant visa then they were already found admissible to the US. Some of them were just straight up illegals (like dreamers). I've met dreamers from time to time and all of them regularized their status after marrying (I assume the ones that didn't though weren't eager to tell me about their status so I simply never found out).

One interesting note here is the case of DACA recipients. If they leave the country to adjust status it should triggers a re-entry ban unless they're granted parole (DACA are quasi-illegal but granted a form of amnesty as long as they remain in US). AFAIK parole isn't granted for US consular visits, so it's effectively banishment as punishment for trying to adjust their status to reflect their marriage.