i can argue they ar enot even selling 50%, since they don't need that much to cover their bills
Are you sure about this?
If you buy some hardware today, you spend money and that hardware is obsolete in less than one year.
Then there's electricity.
I'm not so sure they are earning so much.
Mining farms are mostly cashing out a very small portion of their mined coins. And most of the money from cashed out coins will be used to buy even better hardware. They constantly keep increasing their hardware to not fall behind since the competition between the Chinese mining farms is very high level. At this point the majority of their coins gets saved offline for when the price climbs up. They don't dump a lot at once as they can easily push back the price to sub $100 levels without much effort.
I repeat: are you sure about this?
If you go to a calculator you can see that after buying new hardware and with electricity cost the reward ENDS after few months.
This means you have to buy new hardware.
I am sure they are making some money, but I'm not so sure they are making millions.
If it's so, then why I can't make the same by buying hardware? Calculators say income will end after 3-6 months, depending on difficulty increase.