Why is it "meta" for melee DPS to do the call-outs and countdowns?

[Mesoud]

That's a really good question, friend.

The reason why is because burst largely revolves around synchronisation to increase its chance at success.
This entails the team members timing their burst as close as possible to one another. Now, ranged players are indeed completely capable of calling things out and usually have a better view of what's actually going on on the map.
The only major problem here is that the highest quality of call-outs are those that are fast/unpredictable to the enemy team/cannot be stopped. If a ranged were to spearhead operations it would largely reduce the chances of achieving successful burst.
Why? Because the have no idea when the melee can actually follow up on it with their limitations in terms of the range of their abilities.
I do however encourage ranged players pitching in if they clearly see a better call and one which is more likely to succeed; just keep in mind that you pay attention to your team and whether they can join in on your call or not (applying to both DPS or anyone doing call-outs for that matter).

This is my personal opinion. Hope it helps!