Vent/rant about unclear speech

[Foolacious]

When I cannot understand someone speaking to me -- usually but not always over the phone -- and I ask them to (1) slow down, or (2) speak louder, or (3) speak more clearly, why is it impossible for some of said folks to do any of those three things? This happens both with English (my native language) and with Spanish (I live in Mexico). Clearly, it's not universally impossible, since many people are indeed able to adjust appropriately. What I don't get is why this ability isn't universal. I think I have observed this disability most frequently in adolescents and young adults, but I haven't actually kept careful statistics.