Why hasn't bitcoin mining been scaled / wouldn't that ruin the concept?
The Bitcoin network automatically adjusts "difficulty" to produce a block every 10 minutes on average. As the difficulty rises it takes many more hashing attempts to find a block, so no matter how many exaflops in the network (we are already over 1) the number of coins will still be around 3600 daily (until the reward halves again.)

The only advantage to scaling is if you can scale faster and cheaper than the other miners, and even they you will only see an earning advantage until the difficulty changes and/or the competition steps up and matches their increase.

Technical Explaination of the mechanism:
https://en.bitcoin.it/wiki/Difficulty

More detailed and descriptive link for newcomers:
http://coin.furuknap.net/understanding-bitcoin-mining-difficulty/