Why are key cap characters typically aligned on the left of the key and/or the bottom-left corner? And why are they typically italicized?
Of the two computers in front of me, the letters on the keys are actually top left. 
Wanna fun prank though? Gently pry keys off the keyboard, rearrange them, and snap them back on–‘hunt and peck’ typers will have a helluva time.
I did this once, and confused the hell out of a crusty old Sergeant. He hated me for two weeks.
Tripler
They’ll flummox themselves by trying to spell words right. It’s hilarious to watch.
Key lettering is Roman (not italicized) and aligned top-left on the mighty IBM Model M, the Greatest Keyboard Ever Made.
I just checked about a dozen keyboards (including one laptop), and all of them have the characters on the upper left corner of the keys. None are italicized.
I suspect that your fingers tend to hit the lower areas of the keys, so moving the letters to the top left helps to prevent the letters from rubbing off.
Having them top left makes sense on a keyboard layout that uses “Alt Gr”. Per convention (and ISO 9995 IIRC) bottom left is the unshifted character, above it are what you get when you press the key with Shift. Bottom right is AltGr+key and top right is Shift+AltGr+key.
On the letter keys the printed symbol is the shifted one and thus go in the top left. The lowercase letters are implied and not printed.
On my mac keyboard the letters are centered on the keycap, though.