The problem is depth. Mold sends root-like hyphae into porous material. Bleach is mostly water, and while it can lighten the surface stain, the water it leaves behind can actually feed regrowth. Within days the mold returns looking like the cleaning never happened.
Proper remediation, per IICRC S500, is physical: contain the area so spores do not spread, remove the contaminated porous material, run HEPA filtration, and - most importantly - fix the moisture source and dry the structure so the mold has nothing to feed on.
Bleach has a place for a small, fresh spot on a hard, non-porous surface like tile or a bathtub. For anything on drywall, wood, or a growing area larger than about ten square feet, removal and drying is the reliable fix.