I recommend you familiarize yourself with the GPL-3, if you build something based on a GPL-3 piece of software, it's not an 'optional' obligation. If you want to build something on an open source something and keep your derivative private, then use something that originated as BSD licensed, as Sony did with the FreeBSD kernel that ended up in the PS4 and PS5.

It is required when a gift comes with obligations (as with GPL-3; it's not a "Gift Public License").

It is morally expected because we would all ideally like to live in a world where everyone understands the spirit of gifting, which will lead to a better, more just world overall.

If something is granted under GPL terms, it's not a gift. It has terms.

You can accept the terms or decline the thing. The terms are simply not money. The terms are "you can have this and do anything you want with it and the only cost is that you have to give the next guy the same thing you were given."

Why is reciprocation such a problem?

It's morally expected, not required. And because that's the spirit of FOSS. It's just considered bad form to extract, profit, and not give back.

I don't know any FOSS folks that think they're "giving people a gift" alone. It usually goes deeper than that. Maybe some kind of anti authoritarian, anti establishment, or anti capitalist ideology, or a strong care for user freedom, or a desire to participate in the FOSS community.