I disagree, you can own a DNS domain and change mail provider, bringing your identity and domain along.
Also, it's not because other services do it that it's a good thing!
With email, you can use gmail, or yahoo, or buy a domain and host it yourself, or pay a provider (including google) to host your domain.
Are there options to host your domain on Mastodon without hosting it yourself?
Also, what are the chances that the current servers/providers will still be around in 2 or 5 years?
As for why not Twitter: suppose you want a private instance for your family. Or company. Or softball team. Mastodon makes it trivially easy to launch an invitation-only server so that you can chat and share images with just those people.
It also means that you and I can host instances with different community standards. Maybe you want to focus on liberal horse-lovers while I want a hangout for Republican fisherman. Mastodon lets us both go our own way, and gives us tools to decide whether our very different servers should even talk to each other.