They have to stay within a certain price point to make $. An insane spec console would fail, as not everyone is rich.
And making few high end consoles is questionable because mass production brings costs down. If you make just 500 top end consoles, the costs probably wouldn't even be covered by the final price. Then again, maybe that could work, having 2 or 3 models of same console. Basic, High and Top end where basic one ensures running all their games at certain level where higher ones add higher framerate and resolutions. Because for this to work the games could essentially all be the same and would work on all consoles equally. I mean, someone with 1080p LCD TV doesn't need a 4K capable console because games wont' really run any faster given they have framerate capped at 30fps in most cases. Unless they'd unlock this as well. If 4K capable console uses 1080p screen, cap framerate to 60fps instead of 30fps. It's relatively simple thing...