Nothing wrong with not being able to donate, it’s not required or anything, nor should you feel pressured to if you do not want to or cannot donate. I often do not donate monetarily, instead donating my time, code, etc, instead.
I think the reason Godot, Blender, Krita, and others use a monthly donation system is that it makes it easier to plan how they spend the money on hiring developers. Hiring requires at least a semi-consistent income rate, and using a system like Patreon can provide this. Its not a perfect system, but when combined with corporate sponsors and contracts, it can work.
I’m not sure for Blender or Krita, but Godot does also take one time PayPal donations as well, its just not as visible on the website because it’s only on the Donation page. If you want to donate but without a monthly membership, through PayPal is probably the best way.
