Slippery-slope fallacy. The money to buy shares has to come from somewhere and the power of the purse is with House of reps. Someone like Trump can (and is-per this post) take stakes in random companies, but that's our democracy. You wouldn't say "bomb canada, france, england and norway" because the military bombed one country right? You make sure congress checks and balances that power.
If the government takes over those companies and they don't do well, it means lost jobs which means lost elections too. There's a risk calculus to be had.
The current policy of never intervening or taking ownership in companies.. unless they are "too big to fail and start failing" only benefits the companies.