Because one strategy returning $50-100k a year is a pittance to an investment bank. And (I bet) those strategies require a lot of manual intervention. Plus the risk value if they go haywire.
Honestly I have very little insight into how the bigger firms work. I've only ever done this by myself. But I can imagine that it's a lot harder to come up with 1000 strategies that don't compete with each other for the same opportunities than it would be to hire a team of geniuses that can build a single high capacity strategy.
How long have you been doing this?
Sounds like you are eyes wide open anyway.
Nothing, I work for one, but we often won't bother with the smallest of the strategies. It just depends on the markets and the players more than anything. They call us Quantitative Trading Firms or Algo Firms or HFTs.