For the same reason why shovel manufacturers don't dig their own gold. Why take the risk of uncertainty that comes with the whole mining operation?

Pain in the ass to run. Competition will torch your facility if you don't spend enough money and effort on security. You're going to need your own power plant, etc.

If you are the manufacturer, why not let the customer take the risk? You are planning and spending years ahead, why gamble more than you need on cryptocurrency price?

i wouldn't be surprised if they do.