because Barack Obama isn't running for President
He certainly gave that election quite a shot of energy but Democrats also came a little shy of 10% in 2006. Putting aside 2010-2014, where Obama being president or being on the ballot didn't come close to wave-like numbers in the House, Democrats don't need to have Obama to have a wave.
Democrats are probably getting overconfident about 2018
(myself included), but I also think Republicans are are underestimating
(often significantly) the chances of them getting wiped out next year. It doesn't take a veteran strategist to see all the warning signs popping up.