Lets start with engaging with fiction is an escape from the pressures folks face in their day to day for a good number of fans. It's arguably one of the most popular means of procrastinating (and this has been true for a long part of human history), it's just internet has made it more accessible and given variety
Many men and even some women at least entertain the notion of having more than one partner in life (particularly those who have either legal or religious restrictions that state only one partner at a time, used to be partner for life, unless the spouse died early enough in the marriage). Harem stories are a way to entertain that guilty pleasure.
The problem with the harem genre is most authors rely on troupes or cliches like fan service, the hard to get or tsundere female lead who is beautiful yet seemingly unattainable to a plain/goofy male lead (this isn't true for just harem, other genres like horror, romance, isekai suffer similar issue because the genre is popular and folks are spitballing ideas to meet demand but this creates a vicious cycle where folks feel disillusioned by the poor quality of works being produced because most authors are writing for demand rather than doing what they are passionate about), banking on no matter what they do, the story will sell well (and when you have to make a living, you do what you gotta do). For every masterpiece of a harem story, there are 100 or more sh.it-show harem stories. For every decent harem story, you got 10 trash class stories. A big part of the appeal is the art, and the storyline is baked in or non-existent (this may be influenced by piracy that many would be customers don't buy works, and the ones who do buy just buy anything with good art. Unfortunately, the industry must follow the buyers who keep their lights on, not the readers who browse.