I know quite a few people who have either not looked for jobs because it would have meant less income overall, or refused promotions / higher salary for the same reasons.

This is a well known phenomenon known as the welfare trap [0].

This may not be the case if you have the opportunity to go from unemployed directly to a higher salary but most unemployed people have minimal wage as a best prospect, many times not even that.

UBI evens it out in that if you do get a job you'll make more money instead of downright trading your free time for busy time with no financial gain.

[0] https://en.wikipedia.org/wiki/Welfare_trap

As an anecdote, my wife used to work in the front office of a low income apartment complex, and she said it was common for tenants to purposefully stay on welfare/unemployment, while getting paid under the table for stuff like landscaping and handyman work. Getting a normal job would have effectively been a pay cut for them.

Unemployment is only a minor part of this simply because it expires after a fixed duration. Housing and healthcare are much bigger factors. For many people, getting a job means losing their state funded healthcare and housing, but the income from the job is not enough to make up the difference.