Why do the Old Gods...

[FreshKamel]

do all this stupid distracting shit on the surface on Azeroth being all like "Hey, we're gonna do the bad thing!" instead of just converting Azeroth quietly under the ground? Are the bad stuff on the surface consequences of bad stuff happening to Azeroth? Huh?

[Utigarde]

Because they need to escape their prisons, IE return to the surface, in order to corrupt Azeroth.

[Flatspriest]

do all this stupid distracting shit on the surface on Azeroth being all like "Hey, we're gonna do the bad thing!" instead of just converting Azeroth quietly under the ground? Are the bad stuff on the surface consequences of bad stuff happening to Azeroth? Huh?

They need the energy from the bad that happens to fuel their conversion of Azeroth.

[MechaCThun]

They can only do so much while be imprisoned.

Through corruption, by their whispers or samples of their power, they are able to further their cause towards being freed.

The distracting portion you reference could be due to the fact that they NEED others to complete their tasks, so the art of distraction and misdirection goes a long way.

Similar to standing next to somebody and pointing away saying, "GOLLY GEE! WHAT'S THAT?" and when they look away you take their wallet out of their pocket.

Also, at this point in time in the game's universe, the Old Gods are more known. Not to the masses, but to a respectable amount of people in both factions, so outright "Old God corruption and conversion" would probably be noticed. More subtle forms of manipulation is needed.

EDIT: Deathwing is a good example.

[Better]

They gain energy through chaos. This is why before all the titan influence Azeroth got the old gods basically sent their armies at each other endlessly.

[Raetary]

spreading corruption and darkness is their nature.
besides, they have to do it as they cant really do all that much imprisoned.

[FreshKamel]

Ah, that makes sense. Thanks!

[Radaney]

They love to corrupt stuff, it's basically their presence and power naturally corrupts around them.

But Azeroth isn't exactly in great shape if you play through the new quest with Magni, they've been doing plenty to her in the meanwhile.

[Deleted]

Chronicles suggest that life on Azeorth allows the World Soul to be stabilized. I'd always assumed this stabilisation made it harder to corrupt so the OG want to spread chaos and death to destabilize the World Soul so they could Void it. Hence all the madness and world breaking. That is my headcanon though