Google failed me on this. Any help?
As always, 'why' questions are a really bad idea in linguistics. You can reasonably ask these three types of questions:
- Historical developments within a language
- Areal / contact impact between languages
- Hypothesized semantic motivations
Googling these three will actually give you plenty of results.
A quick search will reveal the following:
A: Historical and areal developments
The plural politeness (or T-V distinction as it is sometimes called) first developed in European languages in Latin.
It has spread through language contact to other European languages. For instance, English was influenced by French post Norman conquest.
Developments within individual languages have transformed the T-V expression of politeness/formality/social distance in various ways. For instance, making the plural 'you' in English the only form of address, using 3rd person plural in German (and from German extending it to Czech in the 19th century only for Czech to go back to T-V). Or the development of pan/pani (Mr/Mrs) + 3rd person sg in Polish.
B: Semantic motivations
There are several possible hypotheses as to the semantic motivation of the T-V distinction. These may differ across times and places, i.e. the original semantic motivation may not be the one that plays out in later times and places.
The origin of using a plural form of address is hypothesized to be in the address of the Roman Emperor in the time when the Roman Empire has two emperors (East and West). This is also where the Royal We may come from.
Speakers might also make sense of the form as indicating greater value in the plural (more means better) through a relationship of iconicity. (This could be a ex post explanation of some speakers).
The plural form could also be seen as creating a form of distance or indirectness similar to the use of past forms and other distancing circumlocutions in polite requests (I wonder if you could be so kind as to help me). In this sense, using the third person (whether plural as in German or singular as in Polish) would make the same semantic sense.
But note that plural forms can also be used to indicate intimacy between people who do not want to use informal forms as in the use of the first person plural. "Are we ready to have our meal." This can often be seen as a form of condescension.
Ultimately, semantic motivations are just that - motivations. They can develop in unpredictable ways across times, places and languages and therefore cannot be used for the sort of predictions a 'why' question may imply.
This is a common pattern across language phyla. I disagree with Dominik Lukes' implication that "why" questions in linguistics are intrinsically bad. It's true that our models are not precise enough that we can predict with 95% certainty what structures will exist in a language, but that does not invalidate the quest for scientific understanding of the underlying mechanisms of language.
Since honorific plurals are found across the globe, we can reject the hypothesis that they result (in general) from a localized quirk of Latin (possibly true for Europe), for instance. This pretty much leaves the explanation in the domain of pragmatics and figurative speech. The circumlocution hypothesis is the only one that comes to mind which, in my opinion, is at least plausible -- that is the idea that it is more polite to speak obscurely, so as to not impose a social burden on the addressee. By speaking obscurely, you leave open the possibility of the addressee not understanding (and it's your fault, not his), which avoids imposing a burden to act on the addressee. Additionally, using a form of address that refers to the actual addressee plus others creates the interpretation that the actually addressee can simply do nothing, since the request is really made of "others".
This is not mentioned in the linked Wikipedia article, but in Middle Persian texts from the first half of the first millennium AD the king is addressed as ašmā bayān, literally “you (plural) gods”. This rather puts paid to the claim that the honorific use of the 2nd person plural pronoun originated in Latin.