The biggest reason is because we have a strategic multinational interest in remaining a leader in science. China (and other countries) will fund science. We have to as well, or we fall behind. Its national security.
We also have a strategic interest in draining intelligent individuals from other countries and nationalizing them in ours, which science funding plays a major part in doing.
One of the reasons why its complicated, however, is that the University environment has changed significantly in the US. What used to be academically-motivated institutions dedicated to the pursuit of education are now, essentially, just boring businesses, with more middle managers than educators. As one example, UCLA has a $9.8B endowment. Their athletics programs brought in $120M last year (though, they spent more, and the university itself had to provide gap funding of $30M. yup.) IPAM was receiving $5M/year in NSF funding (DMS-1925919). One obviously extreme way of looking at this: UCLA could have funded Terrance Tao's mathematics research group for six years with the money they used to save a hundred million dollar athletics program that's somehow still losing money.
This is a priorities issue for universities, through and through. But, Universities have slowly evolved their priorities to bloat their managerial class, which has forced them into impossible financial situations where the only way out is to bias investment into revenue generating verticals like maximizing the size of the student body at any cost on the backs of no-default student loans, international students, and athletics programs. Research takes a back seat.
I am all for public funding of science, but even many university researchers would argue, as a part of the system, that its broken (for reasons which extend even beyond those I've brought up). That's why I struggle to take a solid side on this issue; I want science, but what I want more is a University system that actually takes education and research seriously.