Amazon has boatloads of money. They can afford to develop ARM CPUs/SoCs internally to avoid vendor lock-in from other companies, and they can develop their entire toolchain internally (and exploit existing Linux software). They're really that big. I'm still not 100% sold on Graviton/Graviton2 being the best use of their money, but I think I understand some of why they went that route anyway.
AMD isn't like that.
Down the road, the existence of Graviton2 instances on AWS will definitely change the server software landscape, and that may be the single best thing for ARM in the server that ever happened. Better than ThunderX2, better than Taishan, better than anything from Ampere.