This isn't universal. German is a counter example -- the formal Sie derives from the third person plural, while it still preserves Du and Ihr for informal second person with singular and plural.

I don't know Spanish well, but it probably contradicts this as well. Usted and Ustedes are not related to vosotros (informal second person informal, not used in the Americas) and take the verbs for third person like German.

It's not anything so specific as a group. It's about distancing or impersonalizing the speech. It's too direct to say "Do you want something to drink?" and instead the phrasing could be "Does the gentleman want something to drink?"

We do something similar with phrasing like "if you would like" or in German using subjunctive forms of many verbs like "I would like" / "Ich möchte" / "quisiera" instead of "I want" / "Ich mag" / "quiero".