On the contrary, there are footbridges between buildings that aren't enclosed, eg Central Elevated Walkway between Hang Seng and 100 QRC (considering Exchange Square section to be a separate section). Or they may connect between "outdoors" part at the top of  the podium (Can't think of an example yet, but Yeung Uk Rd Market has an outdoor hub at NIna Square). Then these may not be `building=bridge` .