Illusion created Koikatsu (Sunshine) and Honey Select (2) among others.
Those games come with a character creator. Thats where the girls come from.
And almost all the assets you see in Lil come from Koikatsu.
I'd say it's closer to half the games on this site once you weed out the abandoned games. New KK-based games pop up several times a week lately.
As to why this matters -- neither Honey Select nor Koikatsu have open licenses, and Illusion was very publically complaining about people making for-profit games using their assets, but Illusion doesn't do business outside Japan directly, so they could only litigate in Japan against people working in the country. If an international firm buys the assets, the lack of an open use license could potentially result in a large number of independent H-game developers getting sued over the use of the games for commercial products.
A lot of developers have been riding a grey area, where "the game is free and always will be, you're just donating money to me so I can dedicate time to the game," but if that ever gets tested in court...
The people outright selling these games on itch.io are potentially fucked.