Replacement is very slow, very messy, and very inconvenient. Millions and millions of square feet of ceilings are repainted each year because they are dirty. Painting is more expensive than cleaning.
Painting covers over the bacteria, fungi, and soils.
Painting is messy, slow and closing off an area is usually necessary. Painting ruins the acoustical value of many ceilings. Painting ruins the fire rating of some ceilings.