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

Are you (and or the torque converter guys) guys that good you nail the first selection and never change them?
Seems to me the race guys would be cycling through these as well as performance guys trying to find the magic ticket? And not to say a gear change or cam change never happens as well!

I am asking because I am curious and also will be looking at a converter change next after my front end rebuild.