An evolution of the demoscene has emerged in the form of a social network where 140 characters of JavaScript code are used to create incredibly impressive and compact demos. Check out Dwitter: https://www.dwitter.net/top/all
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.
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.