Why a noun clause cannot be the object of a preposition?

[Edison_Chen_e_c]

I've got to know the function of a noun clause in my junior high:

That he is homosexual is true. (subject)

All of us know that he is homosexual. (object of a verb) ('that' being a object is omissible)

There's no denying the fact that he's homosexual. (apposition of the noun 'fact')

etc.

I got a crucial conclusion: noun clause is not a clause but a noun. Here is the question, as nouns, why can noun clauses and infinitives not be a object of a preposition?

[Mister Micawber]

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Some noun clauses certainly can: I know of what I speak. Let's go with what we know..

I can't think of any infinitives (bare or otherwise) that can be objects of prepositions-- but then, it is very late at night here now.
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Native English teacher at Mister Micawber's

[Edison_Chen_e_c]

It was in my morning, there's time zone problem.