Planting trees as a means of carbon capture is snake oil, and I believe the likes of Google and Microsoft are too smart to fall for it, so that begs the question - what’s their true motive for doing this?

Money.

Appearing to be eco friendly reduces the chances of regulations that would make them have to change their terrible polluting practices. Any kind of carbon offsets are a distraction to reduce public or regulatory pressures that end up costing money.