Because you would use a "dry film" silicone lube that won't migrate much out of the guides and onto the glass. Do not use white lithium grease, WD-40, regular silicone, bearing grease, etc. as they can harden up when dried/cold or attract more dirt to the guides and cause further issues.
Also, if you have to pull the door panel I would check the guides top to bottom to make sure they aren't pinched somewhere. If a door mirror gets damaged it can pinch the guide just enough to make it drag more than it should. Had that issue on an '01 recently.
