why the fuck did a buy land that is apparently inside b's land?

It happened on "Round The Twist". In that case the precise boundaries of the lighthouse's plot technically didn't include their driveway and they only discovered this when a nefarious company bought out the neighboring property.

Exogenous shocks to his wealth. By which I mean his rich irrational uncle left him land.

It doesn't matter! Asking why is dangerously close to sunk cost bias - what matters is the situation he's in, not why he's in it.