This is something that can easily be argued. Not every project wants a donation, for one. Management of donations is also not straightforward - in case of GIMP, the project donations are managed by Gnome.

Second point: why show it on the first start? People don't trust or like the software at first, so they would just click X and begin using the software. Some would even think it's intrusive.

Rather, I've seen solutions where the software would ask for money right after it's been useful to the user. Duplicati for example would tell you to donate, suggested amounts, right after it successfully restored a backup.