My smartphone takes reasonably good pictures despite a smeary slightly
scratched protective lens window. (It's not actually the optical surface
of the lens).

I used to caregully clean my 35mm Nikon SLR lenses every time I took the
camera out.

What is it about the optics on a smartphone which allows it to get away
despite such neglect?

Perhaps it's to do with something like the focal length or depth of focus?