Because the money for UBI fundamentally has to come from the upper echelon of capitalists, and they don't want it because it both reduces their hoards of capital as well as relieves the structural stress on the working class that keeps them desperate enough to work for dogshit wages in order to continue surviving, and we do not live in a democracy that cares that 95% of the population would be better off for it because those same capitalists own all major forms of information dissemination, lobbying, and campaign funding.
Which just circles back to the core problem of UBI: if you somehow managed to unite enough people willing to buck the current system to get it instituted against the wishes of the ruling class, then you've also reached the threshold to end capitalism outright and bypass the half-step of making the ultra wealthy give back a tiny percentage of their ill-gotten gains in the form of UBI in favor of just building a society where the collective wealth generated by the workers is spread among the workers.
It's an understandable position to hold for someone who hasn't gotten over the hump of understanding that these issues are structural to capitalism. If you feel that reform is a possibility then UBI makes a lot of sense, because from the ideals that liberal democracies tout it seems like a win-win: workers get money to live off of as automation and outsourcing put them out of work, and capitalists get a more consistent flow of money through the economy to stimulate growth that offsets the cost of UBI itself.
The issue is that capitalists aren't interested in the long-term vision required to build factories that take 30+ years to pay for themselves, they want maximum returns right now and don't care if that tanks the economy in the long term because they'll already be fantastically wealthy and insulated from the effects. They also will not, under any circumstances except having no other option, ever give concessions to the working class, because if people start remembering that's an option they're liable to remember they outnumber the people in charge 1000:1 and might push for more.
That's why the US will not reindustrialize and why UBI will not happen; they'd rather pump-and-dump another AI/real estate/micro financing loans bubble than build anything of real value. When all that matters is the accumulation of wealth society fundamentally breaks.