ARM sells... "intellectual property" that I might try to come up with a distinguishing definition from the IP of "one clever idea I want a monopoly on for a while".

A lot of IP boils down to an idea that you could describe in an elevator and a few good engineers could turn into a product in a few hours to a year.

ARM sells things that represent... thousands? hundreds of thousands? of engineer-years. You can't just whip that up in a short time, it totally makes sense to pay someone else for their trouble to make something... but if you wanted to and had a billion dollars to spare, sure you could do it yourself.

The problem with "I thought of it first" IP protections is the phenomenon of simultaneous discovery. A whole lot of ideas just have their time come, and tend to have multiple people working on them and coming to similar results in short periods. Making this a race and letting the winner sell out to groups that either want to extract rent out of the idea or to hide it in a safe to prevent competition for 30 years is not conducive to progress.