He spent most of that effort undermining proven solutions and propping up his own investments which have a poor record so this is not out of character.
The helped easing up on the resources of Earth with his investment in certain pharma companies and now owns a giant amount of the farmland of the US too.
Any given year congress could pass something letting farmland owned before X date be passed to your children without taxes. I've seen lots of congressmen telling sob stories about a constituent losing the 8-figure family farm due to taxes. Gates owns the farmland personally, not in the foundation. But it could just be diversifying assets. Lots of tech billionaires buy up lots of land.
I was thinking Buffett said that and Gates was still leaving some. I don't know why the farmland then. Maybe to get easy access to veterinarian antibiotics.