UV from the sun is UVA (320÷400nm) and UVB (280÷320nm) which are, especially UVA, less effective for disinfection than UVC wich doesn't reach earth surface as it is absorbed by ozone. UVB can still disinfect but needs much more time than UVC, as in water disinfection https://en.m.wikipedia.org/wiki/Solar_water_disinfection but since its less effective it requires at least 6 hours, while UVC only takes seconds Wastewater Technology Fact Sheet: Ultraviolet Disinfection. Reason for this is that DNA, the target molecule of UV, has peak absorption in the UVC band at about 260nm and that's close to the 254nm emitted by germicidal lamps https://www.bmglabtech.com/en/application-notes/uv-absorbance-dna-quantitation/