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.
@ayin Nothing is stopping Admins to look up all the data you share but Mastodon is not created to be encrypted and secure its all in Plain-Text that can be easily extracted by Admin. Tip don't share personal/confidential information on to this platform for this you can use proper encrypted services. Also private messages are not so private better not to share any passwords etc..
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