The point was to have some cool thing to fight and that there would be some bizarre situation where legs would be better than treads. Kojima is not a guy with actual military experience, the most it gets is military consultants for the sake of verisimilitude.

As a similar example, in COD MWII(the 2022 one) the final battle takes place in Chicago, not because there's a big ass invasion of the US but because they wanted to have the Chicago skyline in the background and show everybody's favorite aurafarming operators infiltrating from the Chicago river, and thus the main villain somehow set up a cruise missile launcher on the top of a skyscraper and is using it to launch said missile from Chicago at Washington DC.
There's zero reason if it had to be fired from land they couldn't have picked some fuck-off cornfield to do it vs the third largest city in the US other than "it looked cool."

If there's no real coherent reason under scrutiny it's because it probably sounded cool to Kojima at 3 AM in the morning.