Science Fiction

If we're aiming more towards realism, there are many reasons no modern military fields anything which looks like a mech. Not the least of which is tall, thin objects stick out on a battlefield and becomes targets. If you want an armored vehicle with a big gun, you build it low to the ground and end up with a tank. More survivability usually boils down to two factors:

1. Lower observability
2. More armor/defense

You don't die if you don't get shot, and if you do get shot at you really, really want to prevent whatever hit you from penetrating in and killing the crew and/or disabling the vehicle.

Mechs, with spindly legs end up high above the ground and those legs become obvious targets given the complexity of making a leg work. You'd want to reduce the height, meaning shorter legs. Then you want to not have something as horridly complex as an actuating knee or hip. So, let's just use a tracked drive or wheel instead. At for the top, why arms? Again, too much complexity, just a single rotating turret would be simpler and easier to shield. That head thing can be reduced to a sensor mast and we'll just make the sensors omnidirectional to avoid the whole "make it spin" complexity. And um, we just built a tank. Sure, there is some advantage to walking vehicles, and they might make sense on a small scale or in support roles where they are much less likely to come under fire. But for a front-line armored vehicle, I'd buy tanks.

At the same time, mechs look cool.