Google did this for the same reasons AWS created Graviton; the article kind of mentions this: lower energy consumption, better performance for common workloads, and improved security.
When Graviton started there were barely any ARM64 server makers in the market (1s that are likely in it for the long game). Intel was stuck and AMD wasn't as much of a thing as well.
Intel / AMD has vastly improved and ARM64 has been more commonplace (both GCP and Azure do have ARM64 server offerings).
Right now the most important factor besides cost might be ensuring supply. With the chip shortages there was a huge backlog and who knows when it might happen again.
"AWS Graviton3 processors feature always-on memory encryption, dedicated caches for every vCPU, and support for pointer authentication."
Further reading on pointer authentication [1].
[0] https://aws.amazon.com/ec2/graviton/
[1] https://www.qualcomm.com/content/dam/qcomm-martech/dm-assets...