Dropbox is in a relatively specific situation wherein their cloud costs would be extremely high (storage, bandwidth) and their in-house technical skills are probably quite good.
It's about their core values and identity, at least because in this way they are seen in the market as a big player and not just as another AWS reseller with some added benefits.
it would be insane for them to deliver their video content over aws.
By some measures, that's much more extreme than running your own datacenter.