I think it's just a tactical problem. A lot of captains don't feel like they have the resources to attack for more than half the game and it's probably a true assessment for a lot of teams.
However, in the first 50 overs of a Test innings, a single bowler might get as many as 20 overs. 11-15 overs is a pretty standard effort. One-day captains don't have this capacity to press their advantage and protect their weak links; hence the stalemate.
If you could have one guy bowl 15, then the 5th bowler would only have to supply a spell of 5. You could reasonably allow 3 bowlers to bowl 12 and just ask that someone is prepared to bowl a T20 sized part-timer spell; which is not to say that it could never be more, but nor is it suggest that there aren't already some very tokenistic bowlers being treated as something more.
It affords captains that ace-in-the-hand pressure to make batsmen accountable as well as the real advantage of being able to get a couple more overs out of a preferred bowler. In turn, bowlers get a chance to have more success when it is their day. It's not all bad for batsmen either, because the selectors might just pick one more batsman because he can bat, not because he can kinda do both.