Then why are DuckDuckGo results also (arguably even more so) polluted with spam/scam sites? I doubt DDG is making any profit from those sites since Google essentially owns the display ad business.
If you own the largest ad network that spam sites use and own the traffic firehose, pointing the hose at the spam sites and ensuring people spend more time clicking multiple results that point to ad-filled sites will make you more money.
Google not only has multiple monopolies, but a cut and dry perverse incentive to produce lower quality results to make the whole session longer instead of short and effective.
Google results are polluted with spam because it is more profitable for Google. This is a conscious decision they made five years ago.