SGML (ISO 8879), the markup meta-language on which HTML is based, has that, as well as many other things, such as type-checked templating.

Hence HTML as a mere vocabulary making use of SGML's facilities didn't need to define its own ad-hoc mechanism.

Btw the <?php ...> construct is also derived from SGML. In SGML terms it's a "processing instruction" rather than an "entity reference", the latter being the basic means for content inclusion.