It's a common error to assume it's power that's required for MAME. The emulation software core has to be developed over time too, so even with astronomical specs it would be difficult to reproduce the kind of playability you're thinking of as the emulation core needs to be developed.
The MAME team strive for accuracy before speed - this means for many releases certain games will not be even remotely playable whilst they achieve accurate emulation - this does not mean a game will run fast. Once they believe the code emulates the hardware accurately enough they will change code to improve performance - this is a process that may take many years, even with the advances in PC performance. Back when .chd games were first introduced it took well over a year and many subsequent releases before games like Killer Instinct became playable, and that was coupled with the fast-advancing PC technology then.
What you have to bear in mind is that the newer games are they try to emulate, the more complex the emulation software code has to be, the longer it will be before accuracy reaches acceptance levels and the bigger the initial spec demand will be. Also bear in mind that part of the MAME devs policy is to not push too hard with new games as they are potentially still out in arcades and hence still earning profit and liable to attract unwanted attention to the entire project.
Bottom line is using MAME is not like trying to get performance from a PC game - think of it in this sporting analogy: "the goalpoasts keep moving, the players get subbed, and the strategy changes with each new opponent" and you'll see why the status quo is what it is.