Those engines provide features that PHP natively lacks, or doesn't support well. With raw PHP, you have to manually escape every variable with htmlspecialchars, or mix HTML and PHP contexts in a way that can make it difficult to reason about complex templates, (see: Wordpress) or separate concerns (anyone editing templates now has to know how to write PHP, because templates are just PHP.)

A template engine can give you context-sensitive escaping, template caching and inheritance, macros, and other features that increasingly become necessary for modern web development, and many of which you would probably wind up implementing anyway, once a project reaches sufficient scale and complexity. While raw PHP is arguably faster, it's also more difficult to do properly.

Also, culturally, the PHP community isn't as centralized as other web development language communities seem to be. It doesn't have one 'correct' framework or one 'correct' templating language you're expected to use, so there is competition between multiple engines.

But the end result is still the same in either case, basic CRUD - taking an HTTP request, getting some data, pushing it into a template, and sending a response back.