Mainly because all the harem stories like to put a hot chick on the cover instead of the protag.
People do what will sell them more copies.
Attractive girl on the cover, will attract more eyes and as such more potential readers, than sweaty ugly bastard MC.
That said, I would never read solely because of the cover. Or at least haven't encountered such cover yet.
But the cover may decide if I will click on a story to see its description or not.
I know that not everyone has skills or software to create a cover for himself.
And MSpaint cover might be completely fine.
Just remember that this cover will be much smaller on the website and your letters are really thin.
They might become hard to read in the end.
(If you decide that you want help with a cover I can try to help you according to my limited skills.)