Why do people buy bitcoin instead of doing their own cryptocurrency free?

You can. And people do. There is no minimum adoption rate.

Currency makes even trades of two people better. Though a single in-out legder would suffice it most instances.

The thing is that currency needs a publcly (growth rate of ) known total issuance. The longer a currency is around and the more transactions it helped cattalyze. the more we can be sure about, the initiator not messing shit up.

For free is not completely true. You probably need an expensive sunk coat peacock tail Spencian signal in this market full of lemons.

So why not try it. Try to get a feel for why and for what amount people would buy your currency.