This can be seen in any product (cars, smartphones, food - the list goes on).
From time to time there are designs that don't look like others, and those may or may not be met with approval. If they're liked, those designs (or aspects of them) get blended with the standard.
Yes. This is quite similar to asking why all newspapers look the same.
The USA Today was the first to use color in the masthead (and on every daily paper's front page), and that has been copied by some.
Comprehensive macro changes usually only occur when there is a rather extreme inflection point in technology that makes a true leap possible. Something that was previously impossible not only becomes possible, but makes the experience a lot better for the end user. That's why the iPhone approach murdered the old flip phone industry so rapidly, that technology inflection made a lot of new things possible that were very beneficial / desirable to end users. It justified a large macro change, which users had to adjust to.
Similarly, if the people paying the people in charge of designing a product perceive a similar product's design to be the best, then they will pay those people to copy it.
Chirality, side of the road, alphabet, notational conventions, AC/DC, power frequency, mains socket design, measurement standards, sugar and DNA handedness, etc., etc.
So for a generic website which cares about first-time visitors (as opposed to a specialized tool that people use often) following established standards and familiar UX paradigms is good usability design.