Adding support for a CPU architecture is often a considerable amount of work, and the pay-off is often small, although this of course depends on what kind of software you have. For typical applications for end-users the cost/benefit ratio for supporting anything other than x86 is typically very low. For other types of applications like servers it's higher, so they often do support arm, s390, ppc, etc.

Supporting jpeg however is very low cost, with very high benefit: about 20% of browsers don't support webp. It's a small nuisance, not a substantial amount of continued investment of work like supporting a new architecture.