We have them? Graviton instances are priced pretty competitively compared to similar x86 EC2 instances. A lot of people don't use Graviton though (myself included) for a number of reasons. A lot of software hasn't been ported over to aarch64 yet, and even when it is it can lack the optimization that x86 enjoys (especially with programs leveraging AVX et. al). Furthermore, the CPUs that Amazon use seem to be bottlenecked by low IO bandwidth, which makes for some janky benchmarks when comparing simple database operations.

I think we'll eventually see a return to RISC architectures, but ARM's value proposition hasn't made a lot of sense on the server. It's still encumbered with proprietary licenses that make it extremely hard for CPU manufacturers to compete in the server market. It still hasn't figured out all of it's hardware-acceleration quirks, and you don't get any Rosetta 2 fairy dust on Linux (not that you'd want to use it in prod anyways). All in all, x86 is still the set-and-forget king, and probably more stable than ARM alternatives. I'm hopeful that RISC-V will finally put x86 in it's grave, but that's going to be a couple years out...