Because that's kind of what Guardians are supposed to do.

This is the problem of not having dedicated servers like normal mmos (I know it's not an "mmo"). Usually there are servers for people who, like you, enjoy to "role play". I am not one of those people, but unfortunately for everyone, we are all kind of lumped together, and are going to have to tolerate different play styles. Them running ahead isn't unfair just because you don't want to. This is a legitimate tactic in most games of this nature. If it bothers you, leave and find a new group that favors your play style. Also, it wouldn't be fair to those people to report them for that behavior when, from their perspective, you are the one causing the issue. We all have to face the fact that the game, while fun, has some pretty major flaws that may or may not be fixed in time. But in the meantime we should either enjoy what we have, in anticipation of what is to come, or turn it off and find something new.

There are priority targets in war.

This game has no semblance to real combat. Play the game, not skip 85% of it.