Player: No reason to buy DLC when you can get mods free.
Game company: No reason to add new content when everyone is adding it free.
Do you even know what mods are? You seem to be under the impression that they add extensive high-quality content, and they rarely do.
I modded DAO extensively, but I also bought a bunch of DLC.
You also seem to be under the impression players all want the same thing. The great thing about mods is that we can each have the game experience we want. For example, I don't like the combat in Skyrim (I think real-time twitch combat is the devil), but I know that BethSoft is never going to change the TES games to suit me, so I use mods (in Skyrim, to give me extra armour and have multiple companions to fight for me). Without those mods, I wouldn't play the game at all.
Similarly, the DAO mod I found most valuable was the Detailed Tooltips mod. Clearly BioWare thinks that players don't want to see the math behind the mechanics, and they're probably right about that. But I do, so I use a mod to get it. Without a mod, I would never get it (despite the efforts of one of the DAO combat designers to document the game on the side - thanks for trying, Georg).
And regarding bug fixes, what if I don't like one of them? With Origin and Steam, it's not always possible to run the version of the game you want. Or you might like some of the new features of a patch but not others. I used a mod to undo some of the changes to DAO made in versions 1.02 and 1.03.
Even assuming that what you say is true, and BioWare would make more effort to fix bugs and add features in thr absence of mods, I'd still be forced to play whatever version of the game that BioWare made. Even if most players thought it was better, I might not.
You're basically attacking idiosyncrasy, and it's offensive.