WordPress is one of the few CMSs that sends a header saying it's running WordPress and otherwise makes it very obvious. PHP is so high on the known list because every other back-end you can't tell what it is.
Not true. Identifying a tech stack involves more than looking at a couple of headers. Almost all of them leave enough fingerprints in the headers and page source to identify them. The tools for identifying what runs a web site have a lot more sophistication than you think, in the same way Facebook and Google don't need to see my driver's license to know a whole lot about me.