Standing up email might not be that hard… but it’s much harder to ensure that your mail will actually be delivered successfully. Plus it’s not a service you can typically afford to go down. Any emails you miss during that downtime are gone forever, whereas even if my Vaultwarden credential vault goes down I can access passwords from a device that has things cached at least while I fix things.
Plus the big providers just treat small mail servers with a lot more skepticism than they did 20 years ago.