Another big reason is that developers want to deliver a specific user experience that is simply not possible with the default page loading functionality of most browsers.
This is my user experience with SPAs regarding navigation.
And then people stopped using PHP.
it was kind of slow, and all the functions were inconsistent, and had a lot of foot-guns. It was easier to write more robust code and model-view-controller code in other languages. (not that it was impossible in php)
unfortunately it's the only actually viable language in the browser.
Evidence?
In my experience, PHP seems to be more in use now than ever.
Conversely, I’m gonna need evidence that PHP is more popular than ever because that absolutely is an extraordinary claim.
That doesn't mean good SPAs don't exist - many do!