since the combine decided to invade earth, humanity was solely focused on its survival rather than continuing their other endeavors like the creation of more modern cars or technology. eventually they became enslaved by the combine and were pretty much just lap dogs that did their bidding, humans couldn't do anything for another few years before eventually being allowed to work in jobs, further advance technology utilizing the combine's tech, etc.
a majority of old human tech was thrown away by the combine and replaced by their own forms of technology, resulting in all humans having to work with old crappy computers and pagers in order to communicate, document, etc. sure we can try to use old cars that should still exist to this day, but why would you want a species that you've enslaved operating vehicles that could be used for manslaughter or something to that effect?
