Lots of bullshit jobs exist for a reason other than those jobs being useful. Politicians want to create bullshit jobs because it helps them get votes. And sometimes bullshit jobs exist to enrich corporate owners for government contracts - just look at almost any military contract where literally billions of dollars go unaccounted for. Or look at most construction jobs in the US that are regularly overbudget, late, and oftentimes not even up to the original requirements. And yet - those people still get paid and the people who created those contracts make lots of money. In fact, they usually make more money by being more incompetent, because they have to ask for more money to finish the job.

The question is always: who is benefiting from this?

I see what you're you're saying, but it's not like UBI is going to eliminate these bullshit jobs.

Yes it wouldn’t eliminate them. The processes that create them would have to be changed or reformed in some way. The bullshit jobs/UBI argument is more about filling in if/when these jobs are eliminated, that be via automation or process reformation.

One of the major sources of bullshit jobs are self-annihilating professions, like lawyers or advertising. Monsanto sues Exxon for a billion dollars, Exxon countersues, they spend ten years in court and then settle for 10% of what they each spent on lawyers. Coke and Pepsi each spend a billion dollars on advertising to cancel each other out.

If people had more "disincentive to work" then getting people to work would require paying them more money. Jobs that self-cancel their own profession could then hire fewer people industry-wide and result in a net productivity boost, since whatever they do in the alternative can't be more useless than what they were doing.