Decentralization doesn't mean hosting your own instance. It's about open protocols. There are hundreds or thousands of email providers that you can use and still send an email to a Gmail user without hosting your own instance.
People cannot find their friends on Mastodon because:
+ They can't search all instances across Mastodon.
+ There is almost little to no one on Mastodon to talk to.
So they just go back to a centralised service like Twitter and they definitely not use email as a social network.
I have found that this is an accurate description of the first impression one gets; this is not true, but we're all so used to recommendation algorithms easing discovery that the impression is equivalent.
Twitter is just fine and everyone knows it.