I, too, had wondered; this seems particularly common in the US. My best theory is that in rough handwriting, n and u/μ can look pretty similar, and everyone sort of agreed to only use u to reduce ambiguity.
47 -> 47nF (if next to plain double bar symbol with no polarity markings)
47 -> 47μF (if next to polarized cap symbol)
0.47 -> 0.47μF
47p -> 47pF
10np -> np -> nonpolar -> electrolytic -> 10μFIt's not customary to call a 0.1μF bypass cap 100 nF.
If someone wrote in 100 nF for a decoupling cap, I would tend suspect they are trying to discourage a substitution like 82nF or 150nF and want to find out why.
Whereas if it is 0.1μF, I know we can stick in a 0.2μF, if that's what we have on hand, and everything will be fine.