Because your vehicle weighs eighty tonnes, thus contradicting the use of your claim of lightweight materials, and your subsequent use of an MBT as an example. Things that are splayed out to the side don't work, with spiders being a premiere example. They can't get bigger than they already are, due to, as you already mentioned, the square-cube law.

Splayed out legs [i]do not[/i] work, simply due to how weight gets distributed. Like, something the size of a man would be acceptable, but nothing larger than that.