[necovek]

GPL exists because nobody can (should?) stop users from modifying software they use (at the very least, on the machine level) and such modifying should neither be illegal nor hard (thus source code).

The "sharing" bit is on top, and only comes out if you distribute your changes too. AGPL fixes the flaw where "users" don't really get a copy of the software they are using distributed to them in full (eg. only part of it with JS in the browser, but backend stuff is hidden).

So the focus point of GPL is use of the software (thus users), not writing of it (developers). Developers embrace it because they are simultaneously users of the software written by others, and they are best positioned to make the most out of those liberties ("standing on the shoulders of giants").