Because Trademark and Copyright last
FAR longer than Patent.
- Patent: 15 or 20 in the US, depending upon type. (Design 15, utility 20)
- Copyright,
- for works written in/after 1978:
- Single author Life+70
- Multi-author last alive life+70
- Work For Hire, corporate: 95 from publication or 120 from creation.
- for works between 1963 and 1978: 95 years if copyright mark included or registerd
- Trademark:
- Registered: 10 years, no limit on renewals, provided it is defended and in use.
- Unregistered: as long as defended and used.
- Service Mark: same as trademark.
- Trade Secret: No US national protection; some states have some protections.
Note that due to the much shorter copyright durations of the first half of the 20th C, most of REH, HPL, Zane Gray, and other pulp authors works are in the public domain, but that most of the 80's and 90's lit won't be released into the public domain until the final years of the 21st Century... and no RPG from a major publisher will be, either, as they're mostly work for hire.
D&D OE goes PD in 2064...AD&D PHB in 2073... but most heartbreakers (being copyright their authors) from say, 1980 (when the first few really started to matter) have authors still alive... assuming no changes to extend it further.
Fred Hicks'
Fate? He's not dead yet, so Jan 2095 if he dies next month... if he makes it another decade (and demographically he should get at least that), pushing into the 22d century (2105)...
Now, there is the workaround of rewording/paraphrasing. But that's closed a few games to international audiences. (France and Germany, with their creators rights, consider paraphrases violations, too.)
The Tabletop RP and War Gaming Industries have reacted with a mix of open licenses and fan use policies... so it's clear a lot of creators are NOT precious about their rules additions.