Nothing stops other ARM manufacturers from doing that, but it's expensive, and most manufacturers don't really have a reason to work this hard at it, they have a fairly low-margin biz and their clients aren't going to pay big bucks. For Apple however, these chips are completely structural, they already underlie huge amounts of their business and with the switch of Macs to ARM are slated to do so even more.

It may not be possible for x86 though, because of the architecture's complexity limits e.g. how wide you can decode, and more generally how much hardware you can keep fed concurrently.