As icky pointed out, the passive cooling is a big limiter on the processing power. The 1518 from Siemens is about a foot wide, and from what I've heard the reason is that it needs a massive heatsink to run at the 1ns instruction time they advertise. Even if you add gobs and gobs of memory, your program can only execute so fast. I think PLC manufacturers feel that there is a certain maximum scan time that most customers will tolerate, and it isn't profitable to add too much memory past that.
As kvogel, many PLCs come with expandable memory today. Siemens makes you use their own SD cards, but you can get a 32GB one, if you want to do lots of data logging or recipe storage.
You ask about IO update times: Most PLC guys WANT that input filtering, to protect against spurious signals. The ones that want fast data buy high speed cards. They can get pretty fast; I've seen analog cards that can update faster than 50 microseconds.
A number of other posters list some other requirements, like industrial robustness, etc.
To me, it really comes down to this: if there was enough demand for it, at least ONE of the PLC manufacturers would have products like what you're looking for. Instead, profitability is all about trade-offs, and they just think there isn't demand.