> Tanks are heavily armored underneath.

Not a truthful statement.

They have very little armour underneath, and what they have is just basic metal - nothing technical. They have armour in the front, primarily.

https://i.redd.it/4enb88un2de71.jpg

This was a big problem in Afghanistan, and why vehicles designed urgently in that era look completely different to what we were using before.

To explain more: weight devoted to armoring the bottom is weight that's not devoted to armoring the front, which is much more important for high-intensity combat. And since the bottom has more area than the front, a given weight of armor will be thinner on the bottom than on the front.