I can understand why they didn't do this - literally EVERY player's code is heavily reliant on creeps being a certain way, and now a lot of code also depends on power creeps being a certain way. If you're the kind of person who messes with prototypes, first why? secondly, changing this to a base class will likely break your code - it's not something that can be easily deprecated. Also, there's inactive players you need to think about - if you leave the game for two months and come back to find everything has been broken for a month, I'd be pretty pissed and never come back.
On the other hand, I totally agree with what you're saying: creeps and power creeps should inherit from a common base class, not just for simplicity, but as a general rule of programming too. I'd definitely prefer my code to simply "work" with little to no modification once I reach the power creep stage.
The real problem is that this isn't a cut-and-dry issue. There are nuances that need to be considered at all levels. I can totally understand the player base wanting something, and the developers simply being unable to deliver. They're really stuck between a rock and a hard place here.
But that's my view as a gamedev myself, so I may be biased.