The Grout Is the Problem
Tile itself is easy to clean. Grout is porous, sits below the tile surface where mopping cannot reach it, and absorbs soil, grease, soap residue and moisture until it darkens several shades. Mop water pushed across a floor deposits that soil directly into the grout lines, which is why routine mopping makes grout progressively worse over time. In showers and wet areas, that same porosity supports mold and mildew growth.