Many/most teams don't have heroes at all, they only have noobs :) I.e. most people stay on a team for a year or three and change jobs afterwards - there's no chance of accumulating knowledge. Nobody is sure how things work and why they are the way they are.

I believe many managers are fine with this, because the team is producing something (i.e. "velocity" is higher than zero), but are oblivious to the fact that team could be way more productive if the knowledge was actually retained in people's heads and not just lousy attempts at documentation.