Incorrect. FPGA's are use plenty in high-speed stock trading. These guys play "the game" borderline cheating.
I don't think high speed trading is cheating, and of course they try to be as fast as they can. However, there's the next level where the cheating takes place. These guys see your orders before it hits the floor no matter how fast you are. You can see a bid sitting there for long time, but just the moment you decide to take it, the bid magically disappears in a matter of milliseconds. This could not have happened if they didn't see my order coming. I haven't traded for a long time, but I doubt it is now any different than it was back then.
I am not saying they are cheating (that is defined after all by the market who makes the technical rules), what I wanted to illustrate with that phrase and the TCP story is they will do whatever it takes to get the lowest latency over their competitors. They are competing at microsecond levels of latency. This where their bread is earned, aside from good trading algorithms of course.
This is perhaps one of the weird cases where speed is king
above anything, even if it is at the cost of maintainability and readability of the code. They try to maintain a high quality through rigorous testing procedures and code reviews, while staying "agile" enough that they have a short deployment path to production.
I certainly do believe that their network connection for their trading box more or less runs straight into a FPGA. If you're not in the same building to trade or use a crap network stack, you've already lost so to speak. This is where FPGA's can offer high levels of granularity and parallelism that is very valuable.