Same reason the recently mainlined Mali drivers (another ARM IP block) got no assistance in development from any of the chip vendors. Touching those projects would break their licensing agreements with ARM and risk having to stop producing their chips that integrate Mali (aka most ARM based chips).
In regards to Mali, why can't ARM support open source driver effort? Qualcomm for example are not against Freedreno, to the point that Google wants to make it the primary supported driver now. And Qualcomm are quite nasty in general, when it comes to patents and stuff. So why not the same thing with Mali?
AMD could release a friendlier version but it would not be able to do what majoroty of the market expects it to do.
n.b. there are license-built cpus that have alternative firmware in PSP, but it's also closed and they are not really available on open market.
Had AMD not been broke prior to the launch of Zen, they would have likely built their PSP in house, but they were so desperate to stay alive that they sold the 1st gen Zen CPU designs to a Chinese joint venture for $200 million (which kept AMD from bankruptcy).
Wrt renegotiating with ARM, there is no leverage for AMD. TrustZone is already baked into their chips for the next 2 to 3 years, and if the negotiations go badly then ARM could block AMD from selling any processors for at least a year.
AMD did hire a security consultant (due to the Change.org petition) that told them TrustZone was vulnerable, but this means they need to implement their own PSP which is something that will take 3 years or more (silicon has a very elongated development cycle).
Had AMD not been on death's doorstep for the years preceding the new Zen architecture, they likely would have built their own PSP. Being financially insolvent and desperate to meet the requirements of their OEM partners forced them to license a pre-built PSP.
All of the really super-secret security and DRM critical stuff is in the vendor's hardware and code.