This is just like asking "If apple's computers are so good, why would they bother selling them. Shouldn't they just keep all of them for themselves"

Absurd no?

They are the first movers in this space, each company is battling to become the Microsoft, Apple, (or even Standard Oil) of Crypto Mining.

Simples!!!!Where theres a need, theres a market. Where theres a market there's $$$$$$$$$. Plus it certainly costs them shit tons less to make these things than they sell them for so it's a no brainer! "In a Gold rush it is much wiser to sell tools to the miners, than to get on your hands and knees and prospect"