Simple evidence for that is from the experience of many funds - there are only so many investments that can be made around the world that have higher than benchmark returns.
So its normal for funds to return their capital back to investors as they say "hey, we really cant find more targets".
You seem adamant on assuming that investment = money at work.
This is like saying that pressing on the pedal = accelarate, while magicking away all the work done by the engine, and various other moving parts. The transmission mechanism.
Investment, is not consumption.
It is investment, in order to take advantage of consumption.
Directly providing consumption, is a far better method of improving economic activity, than by investment, in the current market scenario.
Oh wait - maybe I have a bead on where you are coming from:
In the scenario where all infrastructure and all factories were working at max capacity, any further investment in Infra/capex, will lead to an improvement in demand.
In the current scenario though, that is not the case, and adding more factories will not result in an increase in economic output.
Simply put, building a factory, but having no customers is investment with 0 return.