The problem is that mining through the browser is very inefficient. I don't have numbers but it'd probably cost an order of magnitude more, at least, to mine on the browser. And I'm talking about CPU cryptos, forget about bitcoin.
The Coinhive Monero miner is about 65% as fast as a native miner would be, at least in Browsers that support WASM.
Disclaimer: I wrote the Coinhive Monero miner.
Maybe Intel will throw it on their chips as a processor extension.