Since most, if not all, cells in the body replicate and reproduce and none of them merge,

Question

why do our cells let virions in?

Answer 1

Cells need nutrients to grow and divide. They allow those nutrients in via channels or via osmosis. Viruses are small enough that they can get in through those channel, or they can trick the cell into thinking they are nutrients by displaying certain proteins on their surface. The cell reads that protein and thinks the virus is something necessary, and the virus is allowed in. Other cells eat ("phagocytize") foreign matter that they don't recognize. They see a virus and eat it, and then the virus is inside them and can replicate.