It's addressed in the FAQ:

"Does Stripe Climate offer traditional carbon offsets? No. Traditional carbon offsets, while often inexpensive, are unlikely to scale to the size of the climate problem. Instead, we’ve chosen to focus on permanent carbon removal technologies."

Planting trees and renewable energy are some activities that carbon offsets fund.

Not affiliated with stripe here, but I'm guessing because tree planting can't scale up to the level of removal that's necessary and renewable energy is already pretty far down the cost learning curve. Stripe has limited capital compared to govts and large institutions and is being strategic about which projects need early investment to bring down costs.