Keep in mind that there was a leak at Steam and that an old version was set as a release one.
So people were modding it because it has lots of problem, because it's an old version.
Beside, Paradox got a big history of modders started with europa universalis where we were able to create event and modify some elements. People like PI because they can change the game the way they want it to be.
Some want more balance (or the way they think balance would be meet), some want more realism, some are looking for more fun...
You can't make all in one so CO make a choice somewhere in the middle and so people are trying to change the game the way they prefer. Some are changing the sound for realism, and some are changing the balance (capacity change).
This doesn't means they don't like the game, it means they like it but would like other value set.
If I look at all the old game of PI that I have, I think that I didn't play more than 2 games per version of the original game, I always modify some value to fit my way of playing the game.