The US federal government seems to have a largely arbitrary budget. No real experience or rational principle restrains it. We have never seen a clear case of negative consequences related to spending too much, so no one is quite sure how much might be too much. Sometimes a politician sees political advantage in grandstanding about fiscal responsibility, but seconds later, that same politician will be found funneling some arbitrary number of billions of $$$ in pork to their district.
I suspect the truth is that politicians mostly just don't care that much about science because they aren't smart enough to understand its value, or decent enough to care.
We could and should 10x the science budget, and it would have vastly more impact than almost everything else the government spends money on. But we don't because politicians see it as some amusement to fritter a limited amount of money on. Occasionally Republicans will publish a list of ridiculous scientific projects and no one wants to be caught in the crosshairs of that -- even though tens of billions are probably being simultaneously spent on murdering people in a country you've never heard of, or flushing thousands of tons of soybeans down a toilet.
Defense-related science gets an exception to this, which is why a big war is often also a big leap forward for science.