That exists (for crypto mining). https://www.nicehash.com/

Also https://prohashing.com/

I'm actually working on a project that does that!

i guess because the consumers might not always be tech savvy. they just want those items/coins/rewards in whatever game/app they are playing/using. a minor pop-up about the mining stuff wouldnt bother them (although app perf might be affected). that's how i see it.