Why do cells have receptors that allow viruses to invade?

Rather than cells having receptors specific to viruses, cells have receptors specific to molecules it needs to acquire from outside the cell, such as glucose (needed for respiration). It is the viruses who adapt themselves to match one of these molecules to trick the cell into letting it enter. Viruses replicate so quickly that their rate of mutation is very high, which means they can relatively quickly change the shape of the site that binds to the receptor. This rate of change is faster than the mutation rate for a receptor, so even if a cell formed a new receptor to try prevent the virus from invading, the virus would be able to quickly adapt to this change.