As others have pointed out, your cash in the bank is actual money in a sense that (say) Musk or Bezos' wealth isn't - you can actually buy things with it, whereas if they want to sell their shares to buy something else they need to find someone with enough actual money who's willing to trade it for shares which may or may not be worth something down the line.

The really big problem is that regardless of where the money comes from on paper, in an important sense all the big government spending has to be paid for out of that actual money in the form of people's income. Suppose the government carries out a big series of infrastructure projects, or even say AOC's plan for mass upgrading of home insulation. That will use a huge amount of scarce resources like raw materials, workers' time, equipment etc which then cannot be used for other things. Which means that whatever would've otherwise been made with those resources no longer exists to be consumed, and that everyone overall is worse off in terms of stuff they can buy with their income by the amount required to offset this cost. This real tax on the actual real value of everyone's income cannot come out of Bezos and Musk and the other multi-billionaires' wealth, because the amount of stuff they consume is tiny compared to their paper wealth. It has to come from ordinary people. (A similar argument applies to the cost of shuttering all those businesses and paying their workers to sit around doing nothing during the pandemic.)

Now of course, a lot of big companies do themselves use a bunch of resources, but take those away and you lose whatever the company is supplying. No Amazon warehouses? Suddenly a lot less stuff is available to be ordered online. No Tesla factories? Say bye-bye to clean electric cars and hello to more pollution and CO2. Etc, etc.