Most sexy amps with big ass chassis have the chassis as being the single most major expense, yes. When it comes to single expenses, that is.
It is difficult to do it for less. Since it is almost always cnc milled in modern times, thinking it would be better for the end buyers cost in buying an amplifier.. to have the given amp manufacturer use an asian supplier for the given chassis..well...not so much.
The differences in cost are minimal due to automation in machining hardware... and keeping it close to home overcomes any other potential perceived value. Added to that, the given western supplier generally has a a more time steeped set of lore in the given manufacturing, and so on. Thus the slightly higher price gives a potential for slightly better finishing work and overall precision in presentation.
That is slowly changing, but not quite yet. And if it does, keeping it close to home with shorter lead times and better communications easily outweighs any minor costing advantage.
Like Mike Moffat said about the Schiit gear being made in the USA, about board stuffing costing the same in the US as it does in china, so no advantage on that end of things, by going to china. Same for the chassis, for the most part. When making 20-50-100 units of the given ’custom-intricate’ chassis, staying home with the manufacturing makes obvious sense.