For example, the fields you mention (sciences/engineering, medicine) pull in vastly more money that humanities. Because they have more money, they hire more faculty, and they also train and graduate more PhD students.
I knew a few humanities PhD students while I was in grad school, and none of them were funded by research grants. If they were lucky, they got fellowships. That's a stark contrast to engineering where both grant and fellowship funding was relatively plentiful (though I've got a few horror stories there too).
On top of it, humanities jobs are really hard to find (even more difficult than fields like physics that are already known for multiple postdocs, etc.), making it a bad deal all around for PhD students in those fields.
Overall, I think things are already allocated more or less the way you expect, and I don't think we need to piss on humanities for this. They already have it hard enough.
Philosophy of Language would have been dismissed by most making these types of arguments in the late 1800's, but it is one of the fields that was necessary and instrumental in the development of the of the universal Turing machine.
You'd be shocked how many people in the software industry regard Turing machines as some esoteric construct that is of no value and wasn't necessary for the invention of the iPhone or Javascript.
Because learning more about our history and the world we live in furthers humanity in immeasurable ways often only appreciated years and decades in the future.
If you are a medical researcher there is a lot of useful research to do. Most subjects are of questionable value. It is unknown if knowing more about how quarks work will ever be useful.