As has already been pointed out, longer hulls are only more efficient above a certain speed.
Often that speed is faster than the paddler normally reaches
While playing around with Kayak Foundry to design a 15' kayak for my wife, I tried comparing the drag figures for her kayak with an 18' version of the same design.
I did what I could to keep all other variables the same.
It wasn't until 4.5 knots or over 8 km/h that the longer one had less drag. below that the shorter kayak had less drag.
A lot of recreational paddlers spend most of their time below that speed, so it would take less effort to get to their destination in a 15' kayak than an 18' one.
However my impression is that in certain types of choppy water, the longer kayak seems to be slowed less when fighting through the chop.