It's because the natural reaction of a bettor is to take the team at -115 or -125 or even -130; same goes for -1 or -2. They basically view that team as the "favorite" and you're getting them as a virtual PK, when in reality, with home court factored in, that's not the case at all.
You never hear anyone say "this team's +110 -- I'm sold" even though you should.
You never hear anyone say "this team's +110 -- I'm sold" even though you should.