it's because ups fills each driver's truck in the a.m. and the guy goes. his day doesn't end until his truck is empty. that's why you sometimes get a delivery at 8:30 p.m. and that's why they leave a package on the porch and keep going. they want to see their families again sometime.
if the driver is pooped, he's pooped. if he can't say he delivered everything, he can't come back to the garage. they are asking the impossible of their drivers. don't blame the driver. blame their system.
also, they pay so low that often the jobs are filled by college kids, and if they don't empty those trucks, they don't make their classes. ups tells them it's 4 hours of work, but if they deliver everything and wait to get signatures, it takes forever. i don't think they hire full-time anymore to save on benefits. at least here, where there are colleges. for the drivers, it's almost a lose-lose situation.
but it's a job.