Welcome to Xsyon, John.
You don't have to join a tribe, but you will loose the benefits of tribe membership, like permanent housing, storage, tribal quests, trading, etc. Solo players won't be able to build lasting structures.
Some tribes will require more participation from their members, some less. I would suggest checking out a tribe like Nomads if you want these benefits. But no one is going to force you to join a tribe, that is your decision if you feel you don't need the benefits tribes offer.