I think that the problem stems from the isolation-centric mentality that pervades in most gyms, and that's something that started in the 70s when bodybuilding was catapulted into the mainstream. Suddenly 'legs' are just another body part that needs it's own training day in isolation and that makes them easy to neglect. Nothing wrong with isolation exercises but they can't constitute the bread and butter of anyone who really wants to be strong.
Functional strength is correlated to a) the posterior chain: the glutes, the hamstrings, the vastus) because that's where all of your explosive power comes from, and b) the ability to transmit that strength through an stable trunk. This is true whether one is playing football, doing Judo or throwing a discus. It's almost impossible too train these two aspects of general fitness without doing squats or deadlifts, but people either do them improperly or don't do them at all, thinking them dangerous and obsolete and something that can be replaced with further isolation (usually done as an afterthought after the 10000th biceps curl).