As someone who used to have some "Nice Guy" tendencies, I think they are doing a few things wrong.

First of all, they're hypocritical about standards. Nice Guys are usually low status men, yet they tend to pine after high status women, or at least women who are well out of their league. Not only are they wasting their time, they are being hypocritical for judging women for having standards when their own are usually even more snobby. The Nice Guy method fails even in low status women who lack self esteem though, for other reasons.

Secondly, they don't understand what attracts women. They think they're "always being available" and women are going to value them for being "loyal", but what they don't get is that being too available makes them less, not more attractive. I'm definitely not suggesting they should play hard to get, but what they're actually doing is imposing themselves into the woman's life more than they want them to be there. The woman might think they are cute (in a "non-threatening", not "sexy" way) for being so loyal at first, but after a while they will rightfully resent the Nice Guy and shut them out.

Nice Guys also tend to suffer from what I'd call a "Princess Complex". Just like Cinderella and Snow White believed they needed a prince to be complete, Nice Guys think they need a beautiful manic pixie dream girl (who takes the psychological place of a Disney Prince) to be happy and content. Low self esteem is not sexy and most women do not want the burden of being a man's "everything".

Nice Guys can become straight up villainous when they lose their shit on their infatuation and start becoming abusive towards them, or they try to sabotage any relationships the woman might be in, or they try to bring their successful male friends down to their level by ruining their relationships.

If you sense you have some of the characteristics of a Nice Guy (or, less likely but possibly, a Nice Girl) - I would suggest a few things. Don't fixate on one person at a time as a romantic interest, interact with people you're attracted to organically and shed any ulterior motives, find contentment being single and come to see a relationship as a nice bonus in life you don't actually need, and be more aware of how high or low status you are (if you are a Nice Guy, it's probably pretty low) and either just become content with your low status or work on ways to improve it (honing your social skills, becoming more physically attractive by getting in shape/dressing nicer, going out more, etc).