Because Azure
customers want Google to be able to index their sites.
Would you host your e-commerce or social media site on a cloud provider that blocked Googlebot?
It's not Microsoft giving Google a handout out of the goodness of their hearts; it's Azure customers demanding that functionality. (Those Azure customers also don't care about a random little search startup and probably don't want to pay any egress fees to serve traffic to it.)
AWS offers the same: https://docs.aws.amazon.com/waf/latest/developerguide/waf-bo...
Oracle as well: https://docs.oracle.com/en-us/iaas/Content/WAF/Bot/good_bot_...
Akamai as well: https://www.akamai.com/products/bot-manager