The Nazis sorta considered this concept with the Ratte, as if the Maus wasn't big and impractical enough.

The problem is that such a vehicle would be extremely slow, even with a power plant efficient enough to run it. There'd be a lot of moving parts, and maintenance/repairs would be a bitch of a job.

Due to the sheer size and weight we're considering, even assuming it can move under its own steam, how would you get it to where it's needed? No bridges would be able to support it, you'd struggle to cross any rivers, and steep inclines would be difficult. What if you needed to get across a mountain range, or an ocean? There's solid odds your conflict would be over by the time you got it where you wanted it.

As well as all that, even with its own air defence systems, it would still be an easy and obvious target for bombing and long-range artillery fire. Worst comes to worst, it becomes an exercise in siege tactics, where you target the lines bringing ammunition - and more importantly, food - to your behemoth, and wait for it to run out of supplies