Because there is plenty of information above that. The frequency plot of the speaker itself doesn't tell the whole story (which isn't even measured in a cab). All the air, presence, and a lot of the definition is above 5 k. Cutting all the out will most likely sound muddy, distant, and have trouble cutting in a dense mix.
Here is a 4x12 loaded with V30s, miced with an SM57. First totally unfiltered, then with a cut at 5 k. Look at all the information that's being omitted. It's the guitar equivalent of having somebody talk while covering their mouth with their hands.