Because printing more money is just another form of stealing.
And more government spending is just running up debt that must eventually be stolen from future taxpayers for some piece of infrastructure that people are, apparently, unwilling to pay for. If we need another airport (in a free market anyway), prices for using an airport would increase until its clear a new airport would be profitable.
Government destroys that ability to know if things make sense. Instead, its completely politically driven and waste and unnecessary debt is the result.
The gold standard makes inflation impossible in the long run. The deflation back then was simply a correction of the inflation created by the Fed with its low rates since 1913. A situation made worse by creating industry cartels that prevented any recovery until they were dismantled and many other bad policies.
Now there is no mechanism preventing government from stealing your money. That is not a good thing. And the final removal of the gold standard by Nixon allowed inflation to destroy a great deal of wealth.
The deferral of consequences Keynesian enables, coupled with a corruptible currency, only allows those consequences to build and build. You can respond to each bubble collapse with a bigger bubble, but eventually you can't.
All of these things are just tools for making government's destructive impact larger, and for deferring and confusing the causal relationships. Its all just a sophisticated way of doing what the Seven Samarai were hired to put a stop to. And a whole bandit class has evolved as a result.
Its this requirement for confusion that makes it hard to apply Keynesian to a simple small group situation. It always requires a large economy with decision removed to a special class and bodies that operate in secret so that people can't discern whats happening.
A group of 20 people could readily detect inherently dumb things, such as borrowing large amounts of money from the neighbors to build things that the people aren't willing to spend money on so cronies of the decision maker can have more money at everyone's future expense.
I wonder at what population Keynesianism magically starts to work in his theory. I suspect it is simply whatever size enables obfuscation.
It most definitely cannot operate without some group of industrious people that prepare for the future and who are also dumb enough to lend to people who don't prepare.