Even if Ampere could afford to undercut the internal development of these hyperscalar ARM chips (which to my knowledge they cannot), there is more value in in-housing ARM chips like Amazon / Microsoft are doing: you can more easily try out wacky ideas in chip / firmware design.

Often times these companies might have an idea for how to save power / tweak cores in a weird way on a CPU, but have a chicken-and-egg problem with not knowing exactly how much it'll reduce COGS. Not knowing that and having to negotiate with a vendor (who will often want to charge more for the feature) means that it's difficult to do unless it's an obvious slam-dunk.

By bringing chip development in-house, adding new features skews more towards a political decision that requires less rigorous financial calculations (e.g. "how much power will this save relative to the vendor_cost++ and our own developer cost?" for 3rd-party chips). It basically allows these cloud providers to ship new features more quickly in their CPUs.