Our current era CPUs are only 64 bits. Why no upgrade to 128 bit CPUs?
This upgrade would be trivial and easy to implement, as it would entail merely increasing the byte length of registers. So why have 128 bit CPUs not yet emerged?
Likewise with ASICs. If it is possible to increase cryptographic function on chips, simply by extending the bit length of registers. Why have we not seen ASICs with registers that can hold a bazillion bits?
Think of engines in cars. If exotic luxury cars are known for their V-12 engines. Why not produce V-24, V-36 and V-48 engines if the goal is to produce greater horsepower and torque?
Can it be said that, at a certain point, simply adding bit length to chips and cylinders to engines produces diminishing returns.
There are two reasons for it in peoples minds. One of them is exactly what you said, the returns are not greater there and in order to be greater they are working on it as much as possible and we do get better tech here and there when someone figures how one thing works out.
However, another side says that "they are building them, but not sell them just yet so the previous ones are sold" so basically it's possible but they want to first sell the previous generation stuff and then promote the newer one. I am not saying either is the case, I wouldn't know, I am not really the person to listen to in this case, but I can say that these are the two reasons I have heard.