Question

Why are bathroom tiles usually rough

Answer 1

-- Bathroom tiles are usually cool, so water condenses on them
when you take a hot bath or shower in the room. 

-- The natural result is that a smooth tile would become slippery,
exactly when you're walking around with wet feet and nothing on them ...
a dangerous situation.

-- In order to circumvent this safety hazard, the tiles in the bathroom
should be rough, especially on the floor.