The conjugation of a verb in Spanish has more info about the person: in German sie, Sie und wir are conjugated the same way, and here is the key: they always coincide. Also in the singular, the verb for some personal pronouns very often coincide (ich muss, er muss). So you do need a personal pronoun in German. And well, in English, as you say, for obvious reasons you cannot drop it.
As for French, you do write different, but phonetically there is no distinction between (tu) vas and il va (unless you have some vowel right after). This holds not only for aller.
So, basically, my hypothesis is that since in Spanish from Spain in most of the verbs in various tenses the conjugation fully determines the person, there is no need to keep track of it. Then, although in some versions of Latinamerican Spanish ustedes is used instead of vosotros and the former has the same conjugation of ellos, dropping the pronoun was inherited.
You basically economize without loosing information. There is no information redundancy. (Nice feature, isn't it? ) To support the statement, there is (at least) another language which, to some extent, does the same: Russian. There conjugation is so irregular, that you might omit the pronoun. (Example: знать in present is conjugated as знаю, знаешь, знает, знаем, знаете, знают. The forms are all different)
However, you might want to keep the personal pronoun if you want to emphasize the person. Dropping the person would be like in Dutch choosing je as a personal pronoun and saying the pronoun would be choosing jij.
Late Edit: I just realized there is a name for those languages: pro-drop.
 I think the German analogous of "no information redundancy" –although departing from verbs and I'm unsure if that was how it developed– would be that in the schwache Deklination of adjectives you kind of drop lots of their endings, for the determinate article already has them:
ein netter Kumpel Vs. der nette Kumpel (← you no longer need to write netter, for der is already saying that the noun is masculine, while ein doesn't)