I read that the bacteria or virus only lives a few hours on surfaces. why does make up and contacts need to be discarded?

Contacts, make-up: They may survive longer on moist surfaces. And other irritants (dust, airborne chemical contaminants, etc) also may settle on the lenses. Such recommendations are always very conservative, but it's best to follow them. You don't want to take chances when it comes to eyesight, ocular health, etc. There is no need to discard partly used make-up if not shared with other people.