Uneven playing field? Last I checked, none of my addons are secured behind a password locked website -- they're freely available to everyone playing the game. That's about as even as it gets!
Hacking the game? Blizzard designed and implemented the technology that allows for addons.
It sounds to me like your problem with addons is that you're computer illiterate and can't be bothered to figure out how to make them work. If that's not the case, then I'm sorry for the assumption, but that's how you came off.
As for the reason why the community is developing them instead of Blizzard? Look at power auras: Does every player need to know when their trinket procs or when their ability is on cooldown? No. Even so, that information IS provided in the default UI... it's just in out-of-the-way places. Addons like power auras can be configured to your personal taste -- if you want a big icon flashing in the middle of your screen, you can do that! But not everyone needs it and if Blizzard were to put that out for everyone, it would cause a lot of confusion for the people that weren't interested in it.
Not everyone needs to know when someone is taunting but, as a tank, I like to know if pets or DPS or the other tank are taunting. Your average player doesn't need that functionality, so why should Blizzard implement it when some tank/programmer can easily make it himself?
DBM is an awesome addon, but would it really be good for the game if Blizzard just put up accurate timers for every boss ability in the game right out of the box? Hell no! Top guilds would have even less of a challenge and they certainly wouldn't be happy about it. It would be incredibly convenient, but the mere thought of it really bothers me.
The point of addons is to give the player some choice in how they see and play the game. If Blizzard starts cramming all that functionality into the default game, that choice goes away (even if they do allow you to disable it). It's not Blizzard's job to provide the functionality of most addons. Some of them, sure, but most of them are just fine in the hands of the community.
edit: I'd also like to point out, since you mentioned unfairness, that some players (and theorycrafters!) have degrees in mathematics. These degrees allow them to figure out the sometimes-complex math behind which abilities are best and how best to use them. Does that mean that their math degrees are an unfair advantage? How about players who are just more skilled at the game -- is that an unfair advantage?