> 1. People still do software based on the GNU license. What's the difference?

The GPL family of licences are significantly different from Public Domain. There is still the option of relicensing for commercial use, for example, which is moot under a public domain status. Though some¹ treat the GPL as PD anyway…

MIT might be a more valid comparator, so to answer the question from that PoV: Money. Many OSS contributors do it to scratch their own itch, or for some definition of “community”, the cost of contribution is generally low (or feels like free) and they don't need anything back. Some are supported by donations or sponsorship but not the majority. Those in commercial environments are supporting projects (by contributions or sponsorship) that are useful to that commercial interest, so there is a benefit there but no need for direct payment (they may get payment for support and/or consulting services or via subscriptions for a paid-for hosted instance of whatever). Someone making a film of a book, or a licensed sequel/prequel/other, unless they are doing it for love or just shits & giggles like some fan-made efforts, generally needs/wants to make profit from it, especially in the case of film/TV which can have a large up-front cost - that is unlikely to happen if the new derived work is automatically public domain.

> 2. […] math is not copyrighted, yet it's still being done.

Not for Hollywood level money, it usually isn't :)

> 3. […] Won't society benefit from new stories and characters?

Yes, it certainly would IMO. But it turns out there is less easy money in that. People flock en-mass to works based on familiar IP more than they do to original works, for better or (often) worse. To paraphrase MiB: A person is classy and appreciates original good art, people are a bunch of dumb consumers of fast food for the mind.

Original works do sometimes smash through that barrier of course, they then often become the new IP that a bunch of derived works are based on so in several years time they are part of the cycle makers of new original works are competing with.

> 3. Is it really so important for society that copyrighted movies be based on old stories? […]

No. But it is important for the entertainment industry, for the reason noted above. What is good for society isn't necessarily the same as what people are willing to pay for, and what is good for the producers of works (away from those doing it purely for their own satisfaction or sense of artistic vision) is what people are willing to pay to experience.

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[1] Onyx, makers of the Boox line of GPL violating e-ink devices, to name one of them², see comments on https://news.ycombinator.com/item?id=41412582 for more discussion about that.

[2] I pick them out from that small crowd because I might have been interested enough to buy one of their products were it not for this issue. Unfortunately many buyers are unaware of the matter, or are aware but don't care sufficiently for it to change their buying decision.