Here's why:
* Subject-Verb Agreement: While "es" is the verb "to be" in the third-person singular form, it lacks a subject. We need to know *who* is tall.
* Meaning: "Es alto" translates to "He/She/It is tall," but it doesn't tell us who or what is being described.
To make it a complete sentence, you need to add a subject:
* El hombre es alto. (The man is tall.)
* La niña es alta. (The girl is tall.)
* El edificio es alto. (The building is tall.)