There's something not very dignified or authoritative about an adult male in shorts.

I'd like to see you try to tell that to the professional male basketball, tennis, volleyball, and soccer players who wear shorts as a matter of course. Also, surfers and swimmers. And how about the real-life cops, sherrifs, forest rangers, military drill sergeants, and postal workers who wear short pants? Do you think that they get laughed at and not taken seriously?!

"Because a funny cop movie says so" is not a very convincing argument. ;)

You may personally find short pants silly-looking, but your personal distaste hardly applies to the rest of society.