Estimating how much it will cost you to mine X Bitcoin is not an easy thing to do, especially now that each block almost always has at least an extra Bitcoin worth of fees, with the highest having over 4, meaning the amount of Bitcoin you get per block can vary by approximately an extra 1/3 and this extra value is pretty random.
The power to run the hardware is also constant cost, not a one-time cost like buying a bunch of Bitcoin at market price, so while it's easy to say "Just buy when it's at X value and turn off your hardware", the miner has no way of knowing what that value is, and without knowing the future values of Bitcoin they have no idea if waiting and spending all their current savings on Bitcoin at a low price will actually net them more then just waiting it out and continuing to mine the whole time. It's a crapshoot, but the miners presumably know what they're getting into.
My point was more that it is probably acceptable to risk mining at a bit of a loss if you're confident the value is going to go up, you already invested in the hardware anyway, and you think there's a good chance you may make more then if you had just bought some at what you think will be the lowest price. That said, my example is probably more extreme then would be acceptable.