Why should the sentence be capitalized in the first place? A quick scan on other submissions seems to indicate it is not true, at least for longer sentence. Why

Why should the sentence be capitalized in the first place? A quick scan on other submissions seems to indicate it is not true, at least for longer sentence. Why should a shorter sentence be then? Claiming it is a title wouldn’t win the argument either. As it is not a rule that titles must have title casing. Both (title case vs first letter capital only) are valid typography of a title in English.