Why does Google sell pay-per-click instead of pay-per-conversion ads?

Why does Google sell pay-per-click instead of pay-per-conversion ads?

Google want to show the best content to their users. If you were bidding on conversion it would defeat the purpose. You bid on a keyword, if your content is good for the keyword your quality score increase. In short it works like this; the higher your quality score the lower your cpc -> The lower your CPC, the lower your conversion cost.

They don't want to send their traffic to a website with bad content just because the said website pays more as if it was the case it would result in bad user experience and people would use another search engine.