I'd guess it's because the devs are actually investing the money into what they do rather than pocketing it. They're essentially the only devs I know of that operate like a real game developer. I know that I'm personally not willing to donate to someone that works alone and is already earning 30k+ a month, but I am willing to donate to a startup that's hiring people, expanding and has a vision beyond a single product. Donating will actually affect the end products if they can hire another artist or whatever, upgrade their office etc. Same can not be said for someone doing a VN from home with bought assets. I see it as investing into the adult gaming market and its future rather than donating money to some person already earning a lot of money.
and before someone goes "hurr durr they haven't done anything in years, where's my complete AAA title with flawless combat, rich story and cutting edge feateures", if you look at their development documentation over on youtube you'd see that they do everything from scratch. Every scene, model etc is hand crafted and that takes time and effort. Their team isn't exactly huge so things take time, temper your expectations, be patient. They're not EA with a hundreds of employees.