The trouble with streaming DFDR data is cost. As already mentioned, it is a rarity when the boxes are not found. On the other hand, SATCOM data links charge by the stream volume, usually in tiers, and per connection point.
100 aircraft in a smallish-medium sized carrier's fleet streaming constant data would add up to a considerable sum very quickly.
Something that should be considered is, as moribund as it might be, pre-ditch pings of location data. Other circumstances like multiple system failures, high-G events and other serious situations where a happy outcome might be in question, even if 'minorly' so. Better to be safe than sorry in such cases.