I'll add another argument that doesn't depend on voluntary goodness, just on longer-term thinking: if you can establish a reputation that you're not making it hard for users to migrate away from you, people will be more likely to try your services out in the first place.

And yet another long-term thinking argument: if being able to easily export and migrate data between competing services becomes commonplace, then you'll not only have an outflow of users to competitors, you'll also have an influx of users migrating from your competitors. If you're trying to put a superior service on a market dominated by inferior incumbents, it's in your interest to promote data portability, as - if your service is truly as good as you think - user flow will predominantly go towards your business.