It's pretty cheap to offset. About $5 to offset all the carbon produced during a transatlantic flight in economy seating
That's great, but the environment doesn't care about the cost to offset just that it has less CO2/etc in the air from humans.
Yeah, and the offset is directly causing that much less of the gas to be emitted into the atmosphere by someone else. CO2 is fungible. Rather than eliminating it from the hardest to eliminate use cases first (planes), it makes sense to tackle the easiest use cases first (power generation, trains, and road vehicles). Your dollar goes a lot farther there.