If they collected much more profit from each device sale then they wouldn't have market-dominating IP. Because they'd have competitors who could undercut them.
As it is, a tiny firm of people make an extremely good living off a margin that nobody in the business can really quibble with, and it has without conflict sustained them to do greater and greater work that has changed the world.
You watch: ARM post-IPO will inevitably have to start squeezing more juice out of the market to give to greedier, more transactional, more activist shareholders, and this will fuck up the balance entirely.
An ARM IPO isn't really going to be good for anyone, I think.
They have collected very little profit from their market-dominating IP.