One reason, not yet mentioned, is that large, state supported, universities in the US, have a mission to support the common good. An educated public is essential to that end, providing broadly educated citizens that can drive toward a better future, both economic and otherwise. But that is balanced by limited resources and pushback from taxpayers whose individual wishes don't always mesh well with the needs of the general public.
So, a bachelor's degree is seen as a start, well worth supporting, with further education funded otherwise. Even doctoral students are supported mostly with teaching assistantships, not scholarships, so that those students actually contribute to the undergraduate program in exchange for their funding. Most large bachelor's programs couldn't operate without the support of those TAs. When I was a doctoral student the overall math department had twice as many TAs as faculty members. The TAs were all doctoral students. So, the two, support for undergrad and support for doctoral "fit" together nicely. Without support the doctoral students wouldn't be there and without them the undergrad program couldn't function.
Masters programs are sort of caught in the middle. The students may not have time or experience to be TAs and the funds available are pushed to the lower levels with, hopefully, more impact, hoping that masters students are willing to self-fund for a couple of years in the hopes of higher salaries. This is probably short sighted, but competing interests are at play.
Also, note a subtle distinction. The states, and their universities don't "fund" doctoral students. They fund research and some of that funding flows to doctoral students who participate in it, often in essential ways. And they provide low wage teaching assistantships and (usually) free tuition to doctoral students, but those students have duties to perform in return for that funding. Again, the computation is that this system maximizes the return on expended funds and supports the common good. You can, of course, question the validity of that computation.
Sadly, we seem to be moving to a world with less support for education overall, not just higher education. My own education was richly funded, though I completed degrees in the early 1970's. Things started to go the wrong way (IMO) about then, when we landed on the moon, "winning" a wager against the Soviet Union. It was, I think, the "fallacy of the last move" that suggested that we won, once and for all, and that support for a highly educated public wasn't that important a goal anymore. I've noticed steady decline since. My grandson will have no such luck. It saddens me that universities are dropping such programs as history and philosophy in favor of (only) STEM, and my path was math. A narrow, technical, education isn't adequate to support our future in my view; either for the individual or for society as a whole.