Either it is specific to USA or the data is weird, https://www.codingdojo.com/blog/the-7-most-in-demand-program... reports that in January (based on the same Indeed.com site) PHP is behind C#. PHP is powering >75% websites it just makes no sense to not even be in the Top20
It's because the complexity of those sites is low. My understanding is a large portion of that 75% is Wordpress sites. A very, very small portion of Wordpress sites do much development past a theme (many CSS/HTML) and installing some plugins.
If you have extensive Wordpress work, it's likely better to use a different starting point (and a different language).
I can't speak for the US market, which this data apparently focusses on, but in Germany I think that PHP is still in fairly high demand, but definitely not nearly as well compensated as the more "enterprisey" technologies (eg Java/React)