Many reasons and first would be that mastodon is not designed to generate profit. Plus I would highly encourage to visit https://policies.google.com/privacy then you will be able to make decision if your data is safe in their hands. Mastodon is just very different platform, anyway there is much more to it but policies are always a nice place to start becoming aware of why some services are better from others.
Google works with governments and many other small services dont. I will give you example of service https://www.azirevpn.com/ compare their policy to Googles its way diffrent. Here is a hosting service with a quite good policy https://www.rackend.com/
Also have a look on this website you will find some information
The original server operated by the Mastodon gGmbH non-profit