If your Mercy has to trouble reaching you, your team probably has godawful positioning, it's the role you play and see how dumb everyone is. Like the soldiers that don't take the high ground, soldier is a LOT better on a elevated platform but most of course don't have the brain for that. If your team has bad positioning, you can't zap around, you die. You need people covering the flanks, you need some on main, mostly importantly you need some on high ground that are intelligent enough and get close to the edge.
Of course everyone only knows how to spam need healing and you have to get to them somehow, that's your problem they think, it's better to stay alive than to heal some braindead dps player that doesn't know how to position. playing mercy in pubs is suffering.
Responding to the bolded: oh wow, yes, yes completely yes! I wish I had a dollar for every time I wanted to "Guardian Angel" my Mercy to someone standing on a ledge only to have them be about a foot too far back and therefore the prompt never comes up and I either die or have to retreat somewhere else.
And I always try to dmg buff someone if I am not actively healing somebody. Soldier 76, Pharah and Junkrat are my favs but every once in a while Zarya or someone else will get the blue beam depending on what is going on.