Verbs are conjugated to agree in number with the noun. In Spanish, If you have a plural subject noun, it will have plural adjectives and a plural verb, but the adverbs don't require any pluralizing. Why not if everything else does?
This is not typical of Spanish; rather, it's true for most European languages. It's not so obvious in English because that doesn't inflect adjectives (at least not for gender and number) and verbs only in third person singular. Adverbs can be inflected for comparison (he runs fast → he runs faster) but not in Spanish which uses más, e.g. corre más rapido).
See also Are there languages that inflect adverbs for gender on our sister site Linguistics Stack Exchange.
The adverb as such is always invariable because it modifies the verb. The verb does have number but does not have gender, and the adverb, if it is really an adverb, always remains the same, as is the case with "rápido":
- Él vino rápido. (fast)
- Ella vino rápido.
- Ellos vinieron rápido.
- Ellas vinieron rápido.
There are cases, however, of adjectives that describe the manner in which the action is performed. Grammatically, such adjectives, though placed in the predicate, are said to refer to the subject and are called "subject complements" or "subjective predicatives". As they refer to the subject, they agree with it in gender and number:
- Él come apurado. (in a hurry)
- Ella come apurada.
- Ellos comen apurados.
- Ellas comen apuradas.
If an adverb is used, the agreement is lost:
- Él / Ella / Ellos / Ellas come/n apresuradamente.
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