Presumably because it doesn't matter that much, so it isn't worth the investment for Intel.

It feels to me like people have trouble accepting that both of these are true: (1) RISC vs. x86 doesn't matter that much in practice; (2) technically speaking, RISC is a superior design to x86.

Intel invested in a new instruction set (IA64). AMD did not.

The market chose AMD's offering.

The lesson I take from history is that there is no need to maintain ISA compatibility as long as the old mode is accessible on a process level.

The secret is that Apple controls their hardware/appstore and android apps are ISA independent.

Didn't the AMD64 architecture spank Itanium in the marketplace long before the iPhone became a big deal?

It wasn't Intel but AMD that came up with x86-64. Intel (and HP) indeed wanted to give you a clean 64-bit design - Itanium. And yes, it was meant as a barrier to entry among other things. But AMD won because Itanic didn't run existing x86 software and generally turned out to be slower than Intel hyped.

As for the internal RISC, you won't get it because it changes from generation to generation (also between vendors) and these changes are part of why x86 keeps getting faster. Furthermore, AFAIK these internal ops aren't as dense as x86 to make them simpler to decode and fetching them from memory would make bandwidth more of a bottleneck.