Because by paying a fee to Arm, they get decades of architecture experience, support, peripherals (including a well-thought-out internal BUS), etc. RISC-V has

Because by paying a fee to Arm, they get decades of architecture experience, support, peripherals (including a well-thought-out internal BUS), etc. RISC-V has a good ISA, but there currently isn't a lot of silicon experience in the real world (relative to Arm and Intel, for example). And Amazon isn't going to roll their own to that degree. It's not worth the meager licensing costs. It's like the difference between deploying Red Hat Enterprise Linux and Arch Linux. Each has it's place, but right now its advantages are in much different areas.