I'm not sure the converse is true, that wherever Jakarta Pages is used
Jakarta Standard Tags are used too.
I hear you, and there's probably places at least where Standard Tags are not used, for example if only scriptlets are used. Still, the question is whether that warrants it being a separate spec. E.g. would it have made sense to make the HTTP/2 support in Servlet (I'm just randomly thinking of something here) a separate spec, just because not everyone is using HTTP/2?
There's obviously a tradeoff here; a simpler overall platform, vs slimmer individual specs.
Some of the things I've been hearing with my consultancy hat on, trying to pitch Jakarta EE to engineers, is things like:
"Why does Jakarta need to have 3 security frameworks? Spring simply has Spring Security and that's it". Or "Why does Jakarta EE have two JSON frameworks, my favorite platform simply has JSON, and it works".