As people have already said: Guard can be removed.
'Just use Guard' is not the magic solution that so many appear to see it as.
I haven't personally seen many DRK, AST, AST, AST teams, so I wouldn't want to say too much about those team setups specifically. But premades definitely are an issue.
In a classic DRK-DRG premade, Guard can help, but it's not a perfect solution. You have to be prepared and more aware of your surroundings to ensure you use it quickly enough. A slight delay will leave you still taking some damage after casting, and a single unlucky stun can be the difference between getting it cast in time or not. Not only this, but a notable proportion of your team will be infrequent players who may not be so aware of what's happening and will be too late or even overlook Guard entirely.
These premades are bad enough to vs and all you can really hope is that with warnings/marks, your team can weather the attacks well enough. This isn't often the case, so premades are regularly left with a significant advantage even in this workable setup.
The most troublesome setup I have seen so far has probably been DRK, RPR, DRG, DRG.
DRK vokes, RPR removes Guard, DRGs LB = a mass of dead alliance members, regardless of whether they used Guard or not. The only sure way of avoiding is to keep far away from your group to avoid the voke entirely - and even this isn't possible in all maps, particularly the centre podium of Onsal or the many bottlenecks of Shatter.
Reaper having such a short LB cooldown means a guaranteed Guard removal every single time. And RPRs teleport allows it to get into and out of position extremely quickly, making it almost impossible to stop before it gets chance to LB.
Seeing premades with consistent 350k+ damage, 40+ kills, and 80+ assists and a 90% win rate, shows just what an advantage they have. An advantage which quickly leads to defeatist attitudes amongst some within opposing teams which just exacerbates the issue.