Because metal buildings have side braces that limit how close you can get to the side of the garage without the top of the RV or RV awning hitting them.
Wood Pole Barns are braced differently so you can get real close to the side thereby having full use of the covered space. Go look closely at the metal garages and you will see. Speaking from experience. Like I said I have a metal one and I have to be very carful pulling in and out plus I have to park in the center of the garage to have good clearance on top.
2006 35 Foot HR Admiral Chevy 8.1 Gas, Bought it 2015 and pretty much rebuilt or replaced most of it inside and out. I am a DIYer and have enjoyed the process the last two years. But guess you are never done when you own a RV.