Mildew only grows in between the tiles on the shower wall because it has to do with the porousness of the surface.
Ceramic tile is glazed and therefore not porous like the grout between the tiles. Because it retains water, the grout creates a fertile field in which mildew can grow.
The truth is, though, if you don’t use cleaners that are abrasive and have bleaching agents, you might not have a mildew problem at all.
Harsh cleansers eat away at the grout, increasing its tendency to retain water and allowing the mildew to further seep into the spaces between the tiles.