Why is it that you can't pronounce Vulcans' (star trek) last names?

ok. so i started to wonder why it was that humans can't pronounce vulcans' last names. Like in the episode "this side of paradise" spock tells leila when she asks his full name that she couldn't pronounce it, and in the episode "journey to bable" spock's mom says that kirk wouldn't be able to pronounce her last name(sarek's last name...supposedly).