People will work very hard to increase their income levels above that required solely for basic needs (i.e. UBI levels), see pretty much every non-minimum wage employee.
Many people do work to avoid starvation PLUS maintain whatever level of standards they've become comfortable with.
But the reason IMO people work non basic jobs is because if they are already forced to work their entire lives to avoid starvation, they might as well optimize. If I have to commute, and maintain a car, and be there for 40+ hours a week, I might as well invest in getting the most out of that as possible.
But once people have enough money to maintain their comfortable lifestyle for the rest of their life, many do retire.
There's no a-priori reason why one would expect those who receive UBI to be work-shy.
$3k with tons of free time all week.
Or add working for $3k extra money by giving up all your free time? Not worth it.
Unless of course apartments become $5k a month since that's about the money that a person on UBI plus a bad job has..
Have a job with a free-market rate of $100/hr? Why not work for only 30 hrs a week?
Also, UBI levels are never proposed at $3k/week ($150k/yr), but rather at about $250/wk ($13k/yr), or only 2-3 hours of work per week for the $100/hr guy.