This thread from back in 2010 covers the reasons why many keyboards of the time only effectively allowed 2 key rollover:
https://gaming.stackexchange.com/questions/6669/how-do-i-remove-the-limit-on-pc-...
Viewing post in A plea to developers. Don't only use WASD.
Interesting history lesson.
Once again, historical reasons and cheap tech made people use and stick to the inferior solution. Pinky usage gives me cramps. I can press a and the keys above and below fine, but when I try to rest with index on d, my pinky hovers somewhere between keys or is cramping up trying to find the shift key. While not "natural" I am used to press and find keys with index on f. There is a nice bump on that key to find it while not looking.
Key bindings should be an essential and trivial part of all game engines, so devs would not have to think about it or spend time implementing it themselves. One should think. But apparantly that still is not so.
I think you are missing the point. It doesn't matter how many keystrokes the interface can send if the keyboard itself can only detect 2 keys being pressed at a time. The thread explains that many keyboards at that time could only detect 2 simultaneous key presses not counting modifiers. At the bottom it covers the technicalities of why that was, i.e. keyboard manufacturers going from having each key individually wired to using a grid system to cut costs and having to impliment anti-ghosting due to the grid system's limitations.