Because we've only ever had one Bard in our games. And that character only lasted a few sessions before he was swapped out for a druid. So, Bards have never been a problem because they have never even been a factor or consideration.Swapping Expertise to a dice rather than a flat bonus sounds like a good idea. It would allow Rogues to hit even higher DCs, but also make them less reliable at the lower-level of DCs where the problem seems to lie.
Bardic inspiration dice have a similar effect, even stacking on top of current expertise, however they have not been mentioned as a problem at all.
We pretty much don't like bards and find them--well--useless. That's why I never mentioned them or Bardic Inspiration, and have always focused on rogues.