Yeah when this happened I was on Mercy. Naturally Zen and Ana, even Brigitte and Moira can do this from some ways away, and it's easier for Lúcio to get relatively safely into weird places without flight point requirement Mercy has - but in his case leaving the rest of your team without heals is often not the best choice. However, aside from Moira's orb it still requires direct LoS and waiting for them to actually appear in the distance for that half a second at the height of their jump or when they dash across before they go out of LoS again and land your heal(s) there while trying to take care of others is fucking annoying. If they at least, quite rarely as well, perch somewhere in your LoS to get healing, then yeah it usually ain't hard. But you can't heal what you can't see.
Addition: In fact I tend to spend quite a bit of time obsessively shuffling to try and get Genjis in my LoS when the rest of the team doesn't need me atm, because I actually do want to heal them. Too bad most of them are excellent at avoiding me, more so than enemies (yes I know they probably don't do it on purpose but it's still damn frustrating).