How come there aren’t a bunch of torque converters for sale all the time?

The reason you don't see them for sale is. Who would buy a used converter for a new trans unless you know who is selling it and trust them. When it comes to a used torque converter you never know whats inside.

I once bought a trans and dumped the converter to look at the fluid. It was bright red and I trusted the man. trans was empty. After getting the trans home and pulling the pan the fluid in the bottom was brown with metal in it. The seller emptied the converter and pored fresh fluid in it.

A dirty converter will destroy a trans.