Because in the 1970s, Saudi Arabia shut off oil exports to America, triggering waves of supply side inflation. This manifests both as increasing prices as well as economic stagnation, which mainstream economists thought wasn't possible. The prevailing interpretation was that this was caused by a "wage price spiral", i.e. that
any wage gains by workers will trigger the inflation necessary to claw those gains back.
This is a valid assumption... in a perfectly closed economy where no new energy, material, or labor is available[0]. However, this is not actually the case for most economies - just the one where resource-cursed petrostates are deliberately conspiring to starve out democracy and we decide to let them do that. Energy literally falls on us from the sky[1], we can use it to recover disposed of material, and, if we need more than that, we've got an entire planet full of material to dig up.
In fact, states with lots of mineral or energy wealth already use UBI. Saudi Arabia and Wyoming basically have no income tax because oil and coal extraction pay for everything. Alaska goes one step further and gives state residents a dividend out of the state's permanent oil wealth fund. And, as far as I'm aware, none of these states have outsized inflation compared with the rest of the country / world.
tl;dr economists have a superstition about workers having money
[0] Not to be confused with degrowth, at least I think so?
[1] And when it isn't we have this metal called uranium that literally spews out heat from radioactive decay when you enrich it enough.