Why?
Its simple, because they can make more money when they don't mine for themselves but instead get you to overpay and mine for you instead.
Same reason why Bitcoin Manufacteurs sell ASICS to the public, because they will make more money selling the ASICs then mining themselves.
Only reason why ASICS in 2013 made the public money is because the manufacteurs got their forecasts wrong. But if they could go back in time, they would of never sold the miners until mid 2014 and just mined themselves.
Its all about money.
So when they release new shares to the market after expanding their mining capacity, how do they ensure they sell those shares above the price they bought those new miners at? If the market price is constantly floating, wouldn't the situation arise where selling new shares in the miners you wouldn't be able to recoup what you paid for them if the price per share drops significantly? If that were the case it might be better to just mine with them and take the risk?