because (for some folks, not all)-
they want to retain their jobs. not all employers are holding those jobs for people that simply choose to stay home and collect u/i. u/i doesn't provide healthcare, and buying it on your own can come with substantial sticker shock.
they don't want to end up with a u/i over payment. some employers are reporting those that are simply declining to come back to their available jobs. the u/i administrations in most states are too overwhelmed right now to be dealing with calculating over payments for improperly received benefits but i suspect they will get around to it-and may take a look at the claims that people filled out to see if they honestly reported that their jobs were not available to them for the duration of their claims (u/i fraud is serious and comes with substantial penalties).
they want to retain their u/i benefits for when they truly do NOT have a job available to them, if they have a job available now and exhaust their benefits then their lack of active work history can create ineligibility when they no longer have a choice of working or not.
they want to retain their seniority with their employer which can drive better pay through longevity, promotions and other opportunities (yup, even entry level jobs can have this).
they have a personal work ethic that drives them and perhaps children that they want to model that work ethic to.