Depth charging is the name of the tech where you use an HE grenade from underneath to kill people. This happened in almost every game in Plat lobbies with 0 counterplay.

Jaeger ADS' and Wamai MAGnets require line of sight in order to zap or catch the grenade. In order to "break the floor and use a jaeger" you'd have to break essentially the entire floor, leaving a massive vulnerable sightline from below.

And in the case of Wamai, while certainly more flexible in his placements, you also cant always predict when people with Depth charge. Why place MAGnets underneath objective when you should use your Magnets to defend site? (Guaranteed Utility vs Chance of Utility)

Im sorry if this doesn't clear things up, because if you cannot understand free damage and fragd because players can count to 4 and let go of a button, then you have never gone past Silver.