Like I said, there are many reasons that it's
not a problem- but those are the easy answers on why it
is.
But if one character has flight in a party, I can be sure that as far as bypassing challenges the flyer is going to be ferrying their party members all over. That's where having more actual rules for it would be useful, like those that
@Steampunkette provided. But we don't have those.. as far as we can tell, it's no problem at all for a character to be lifting and carrying party members over obstacles; at least, in non-combat situations.