To narrow the search for consumers, search engines used to measure the relevance of a website or an information page by counting the number of times the searched words appeared on that web page or document. why don't they use this criterion anymore?

The answer is : it was easily manipulated.   To narrow the search for consumers, search engines used to measure the relevance of a website or an information page by counting the number of times the searched words appeared on that web page or document.  They don't use this criterion anymore because it was easily manipulated.

Malware, not sure what else would fit that description.