If i had to guess, many simply want a porn game, others may want a feel good game or wish fulfillment and harem is perfect for that.
At least thats the impression i get, when i see how popular games like WVM are, because it certainly isn't for the captivating story or the deep and diverse personalities of the characters.
As for the devs.
The most devs here are not seasoned game developers, even less experienced authors.
Harem games are rather basic/more easy to write.
U don't need a complex/interesting story, the characters are very fundamental, most of the time the typical roles like, the tsundere, the tomboy, shy girl etc. no deep characters or real character growth, instead u have quantity.
There is no need to make the characters act/behave/interact in a believable manner, because honestly no one even expects that in a harem game, not to mention enough screentime for each character to pull that off.
Of course there are also good harem games with an interesting story/characters, the list is far shorter thought.