There's an implicit assumption in there that reducing taxes will cause people to 'invest' more -- i.e. something like the trickle-down effect. I believe there's enough evidence to show this isn't how things actually play out (people still keep their cash).
Also governments, need to set up/support entities that do figure out how to disburse funds appropriately. Like Research Councils in the UK or the NSF in the US who have established processes to do exactly this. How is that less distributed than a rich person deciding for themselves? Also, there are fundamental areas of research that would be completely ignored if we relied solely on the wealthy to solve problems.
> "Please don't thank the government unless you think they know better how to distribute this money than the people they claim to spend it for."
That assumes that people have enough knowledge about where it should be spent. I strongly disagree with that. You cannot achieve fundamental scientific breakthroughs this way.