It could be a number of things
1 pella windows suck to clean, for any number of reasons
2 Rhino seal, a sealant that supposedly dries like polyurethane and negates the need for painting. I dont know why but sometimes the application just ends up a sticky mess
3 Some self cleaning glass reacts with almost everything, a fingerprint from years ago shows up like it was yesterday, it breaks down your soap and leaves smears and squeegee marks (mostly in the sun)
4 Grease from outdoor grills although would probably not be everywhere
5 Waterproofing, usually only on brick buildings, leaves spots everywhere