Because it can influence the behavior of people who might be going into adtech. If working for adtech becomes known for coming with a cost of harming future career prospects outside of adtech, it becomes far less attractive even without having to adjust compensation. This is also the kind of thing can get associated with moral failings, which can create significant social costs to joining adtech as well.

None of this occurs if you just sit back and encourage people to get out of adtech and make it nice and easy. People will just decide to do it for "just a few years" so they can "make some money."

On top of all that, this sort of change doesn't happen instantly. It'll start slowing and the momentum will grow. People current in adtech will have a chance to see the writing on the wall and jump ship before it's more than a minor annoyance.