Why hasn't bitcoin mining been scaled / wouldn't that ruin the concept? I'm just learning/getting started with bitcoin and one thing that's obvious straight away, is that the more computational power you can throw at it (with the cheapest energy possible), the more coins you can make.

Lots of people mining coins (myself included) just appear to be hobbyists - maybe with some nice equipment, but nonetheless, not 'industrial-scale'.

So why aren't there companies investing millions in solar panels and ACIS machines to mine bitcoins? Wouldn't someone that invested a few million (or more) in a setup like that monopolize bitcoin, in a manner of speaking? (Until people investing equally large amounts?). It'd raise the difficulty beyond what hobbyists could afford to make BTC profitable for them to mine. 

Maybe millions isn't enough, what if someone spent BILLIONS doing this? Would it have the effect I suspect?
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/