It is a matter of responsibility. If you can log into any lemmy instance or mastodon server with the same account, then which server takes responsibility for your actions in the fediverse?
I have seen instances be defederate from because of their lax account creation requirements, or because of harrasment from users from a specific instance.
If an account can log into any instance, then who is responsible for banning the account?
The puzzling part is fun, because you are constantly learning new ways to use your body. See how to balance, how to move around, etc. I have found that dancing gives a similar learning challenge. Especially the more free-form dances like salsa and bachata. It’s fun learning new moves every week during the lesson, and then try to see if you can put them to practise during a party.
And don’t worry about beeing to stiff. If you can balance around boulders, you can get your body to move around for dancing too. Just takes some practise. I currently do both, and feel like I lack dexterity more for the climbing than for the dancing.
And unlike most of the hand-friendly options mentioned already, you do have to use your hands and arms a lot. Just not in a way that puts any stress on them.