Because the browser is on top of the computer. Show me a browser implemented in hardware (not on top of a general-purpose computer) and I might change my mind.
Modern computers are VMs all the way down. x86 is also not implemented in hardware. The x86 machine is actually a VM running on top of the microarchitecture.
Also, browsers are no less general purpose computers than physical computers are. There's nothing you can compute in one that you can't compute on the other.
Why aren’t we seeing browser computers then, if they are analogous as you argue? My point is the reasons why we are not seeing them points to where they are actually not analogous.
1. There's no software specifically developed for webassembly, unlike for x86, ARM, and the other architectures.
2. Webassembly itself was designed by software people, not hardware people. As such, it's probably doesn't make as much sense to implement directly in hardware. Doubly so when combined with reason #1.
3. Webassembly doesn't belong to any hardware manufacturer, and therefore nobody has any incentive to introduce a line of native webassembly machines.
The more generalized aim here is not "run code with the lowest overhead" but rather "do cool stuff within surprisingly strict limitations." It's the same concept whether you're trying to write 256 bytes of 6502 assembly, or 140 bytes of Javascript.
