Why don't we put subject pronouns "you", "I" in questions? e.g."¿cómo tú estás?"

Question

Is this the inherent feature of Spanish? In English we always say who is doing the action:

How are you?

Why don't you say:

¿Cómo tú estás?

Answer 1

In Spanish you can omit the subject in sentences when it is known or it can be inferred from context or the verb. In the case you show, the subject is inferred from the verb.

This is the present indicative of verb estar:

yo       estoy
tú       estás
él       está
nosotros estamos
vosotros estáis
ellos    están

Note that every verb form is different, so knowing the verb form you can infer the subject, so the estás form can only correspond to the pronoun tú, and thus it is redundant to use it in the sentence.

You can also omit the subject when everyone in the conversation knows it, not only in questions but in every kind of sentences:

—¿Dónde está Juan?
—[Juan] Está en su casa.

Nonetheless, there are cases in which you can put in the subject if it needs to be emphasised:

—¿Cómo está tu mujer?
—Regular, aún no se ha repuesto del susto.
—¿Y cómo estás tú?

Here you say tú to emphasise that the subject has changed from the previous sentence, even though it can be inferred from the verb.

Answer 2

This is not restricted to questions, you would say soy Erjan not yo soy Erjan. Since the verb tells us the person it is not necessary to use the pronoun unless either it is ambiguous or you wish to emphasise.

Ambiguous Está en otra planta could mean he/she/you polite is/are on another floor so you might add the pronoun

Emphasis Como quieres = as you wish, como quieres tü = no, as YOU want, not me.

Answer 3

As noted, you can include the subject as a means of emphasis, although I would phrase it "¿Cómo estás tú?" Another example that comes to mind is the classic Spartacus scene. It might be translated:

"Soy Espartaco"
"No, soy yo Espartaco"
"No, soy yo..."

and so on.