Why are some Hebrew prepositions always conjugated in plural form?

Question

(inspired by this question)

Certain Hebrew prepositions, such as אל, על, and לפני, are always conjugated in the plural form even with a singular object: thus אליו, עליך, etc.

Others don't: the single-letter prepositions (ל and ב), and some of the longer ones (כמו comes to mind) don't follow this pattern, and are conjugated in the expected forms (בו, כמוה, etc.). I would have expected that at least ל should match אל, since it's a shortened version of it.

Why is this so?

Answer 1

On further reflection, I guess that part of the answer may be that each of these either is already in plural construct form (לפני) or has such a form in poetic usage (עלי in Gen. 49:17,25, Deut. 32:2, etc.; אלי in Job 3:22 and a couple more places in that book). So אליו, עליך, etc., would be based on those forms.

That still leaves the question, though: why indeed are they so? With לפני I guess it makes sense based on the idea that פנים itself is plural, representing all of the different aspects of the "face" one shows the world; but how would this explain אלי and עלי?