It's a fair question, but in my experience the opposite is true. And here's why & how, in my opinion:
Since you really don't want to mash for any less than 20 minutes, there is plenty of time for the malt flavors to get fully incorporated into the wort, such that waiting for much longer than that doesn't help at all. No one is advocating only mashing for 3-4 minutes like tea. The grains are always in there for a much longer time.
Also, since a longer mash time increases efficiency while a shorter mash time reduces efficiency, then the brewer who mashes for a shorter time generally needs to use MORE malt to hit the same OG, thus theoretically adding MORE malt flavor! And the reverse is true as well, longer mash time means you don't need as much malt, which can dilute your malt flavor, at least in theory. I have run experiments on this in the past that were not conclusive, need to run more.