Because nowadays multiple-word "keyphrases" on a page are hugely more likely to be successful in gaining a profitable SE position than single-word "keywords" - because the competition for single keywords, at least any keyword that is valuable, is so fierce and established that ones chances of getting a single keyword in the top 10/20/50 are usually very small.

(sometimes one can get a new page indexed for a single valuable keyword, but the listing is almost always temporary. Nice when it happens tho.)

This is such an established reality now that most people mean 'keyphrases' when they talk about 'keywords'.

And everyone understands the term 'keywords' to mean 'both keywords and keyphrases'.