I know what you mean since i just re-started a new character. However, the way it is, is a design desicion afaik, and there are good reasons behind that.
I think those reasons are:
- you should be able to come back and still have a challenge
- artificially coupling the enemys to the level would **** with the balance
- coupling enemies to levels would render levels and item upgrades useless
So while i understand that it sucks a bit, i still think this is the better way as it is right now.