Because people like different things.
I'm with the other guy earlier, I don't really see a problem with it. I play in leagues that use D and K, leagues that don't, and leagues that use IDP. It's just a slightly different game is all.
I will say that in the league I'm in that uses defenses, the elite ones have been traded for mid firsts, and for good reason. If you had the Seahawks in their prime, or the Bears last year, you had a distinct positional advantage. Sure, how good a defenses is changes comparatively quickly, but the truly good ones usually put up good numbers for at least a few years, which is pretty much the same for most other non-QB positions. Look at the top RBs from say 4 season ago and tell me how many are still relevant today.
In regards to the Pats D, I agree with commenters above that they will come down to earth soon, but they are still about as close to a "set it and forget it" defense as it gets this year. And while it's surprising they are the overall #1 (and by what margin!), it's not like they haven't been a solid defense the last few years, so this isn't as completely unexpected as say Chark being the WR5, or the entire Minshew situation.