Google is constantly improving the search results. If factors, such as a low bounce rate and high dwell time, show that a site is popular and useful, why not rank it higher?

People tend to think in binary: A site is either spam or no spam. But spam is much more complex. A site can be 90% good and 10% bad. A site can turn spammy without their intention: comment spam, hacked sites. A site can pull out all the stops to hide their link buying. A site can be useful to have in index, regardless of some spammy behavior.