Replacing bullshit jobs with UBI takes away many ways that important emotional needs are met. Feeling useful, important, part of something bigger, respected, etc. These are very important for quality of life.
If we make sure everyone's basic economic needs are met, then maybe they'll surprise you and find a job or social niche where they legitimately are useful and valued. But if you're economically imprisoned to your current job, then you won't have the time or freedom to explore.
Also, unemployment insurance and welfare as they are now are fundamentally different from UBI. The nature of these programs is "stay unemployed or economically useless, or else you lose the benefit" and it actively disincentivizes people from trying to contribute to society.
And it's pathologically toxic that we as a society believe the only valid way to meet those emotional needs is by providing value to a company in exchange for the means to survive.