Why don't people spend Bitcoin more?

We're not there yet.

Bitcoin is just beginning to wrap up its proof to the world that it is the best Store of Value. Yes it has more to go, but the trajectory for that is clear and the world is adopting it at the highest levels.

The energy focus only shifts towards strengthening Bitcoin as a Medium of Exchange once Store of Value solidifies.

The only reason someone would accept it for payment is if it works as a Store of Value. Many Bitcoiners figure this out and once they do, they are happy spend or receive it. The whole world is getting there now.

Square Terminal functionality is a massive first step. Sooner or later everyone will appreciate their ability to avoid 3% credit card fees, chargebacks and inflation. Bitcoin is the best Medium of Exchange in the world. With time the infrastructure will improve and people will begin to use it.

As people get used to transacting in bitcoin it becoming widely adopted as a Unit of Account, but not until then. There is no skipping the time it takes for people to get used to it. It has to proceed this way.

Store of Value -> Medium of Exchange -> Unit of Account

If you already use it for all three, you're living in the future.