Because ...
Kevin Love is (for my money) the 2nd most skilled (offensively speaking) big man in the league. He is a good individual scorer and an excellent team offense player. Love is also one of the best rebounding PFs in the league.
Kevin Love is a lousy defender. One of the worst PFs defensively in the league. Both in terms of one-on-one defense and team defense.
Overall, Kevin Love adds a lot more to his team with his offense + rebounding than he takes away with his bad defense. More of a 2nd option than a true #1 guy, I reckon.