Mastodon can scale, but it costs money. Most instances are hosted by people out of an interest to do good. Servers cost money... And for F/OSS project there aren't big pocket investors ... only funds, grants and hopefully donations.
Many of the mastodon servers I am seeing pause new registration/break down are at a few thousand members, maybe 8k max. I dont know how Mastodon is implemented, but my intuition is that a well written C++, Rust, or even Java service should be able to handle that many users on a single instance.
Mastodon by contrast spreads out this load to servers paid for by users, not by ads.
If you cannot join any servers, pay for a managed hosting provider to spin one up, or host yourself on a vps or a home server.
Most servers will be small, or you can host your own.
It would be cool if someone rewrote it with a Crystal framework. Although there are limitations to this because it's faster than Ruby and similar to Ruby but not completely like Ruby.