You asked: »Why is that?«
Short answer: »It has no special reason.«
Rules for natural languages are not constructed. There is no committee that defines grammar rules. Grammar rules do not define how languages have to be used.
Languages change and evolve. From the factual usage of languages you can derive rules that describe how languages are used.
Remember: Grammar rules do not define how a languages has to be used. Grammar rules try to describe how languages are used in real life.
Grammar rules (i.e. descriptions of the factual usage) can help students and non-native-speakers to learn a language. And think of this: Toddlers learn a language without learning any rule. They even don't know what rules are. They just use the language. And this is why even people can communicate who never attended any school.
This is how languages develop:
One single person begins to use one aspect of a language in a slightly different way than everybody else. In most cases this happens unwittingly and without any intention. In more than 99% of all cases this new way of speaking will be ignored by other speakers. But in rare cases other speakers also begin to use the same peculiarity, and this group can grow. And again: This happens unwittingly and without any intention. Then you have two groups of speakers: A majority that speaks in the old way, an a minority that uses the new way of speaking.
And sometimes this new way of speaking spreads over the whole group of speakers. When this happens, then the languages was changed. This is why living languages are called »living«. They are changing continuously. This happens to all living languages, and it happens all the time.
But the point is: There are absolutely NO RULES that guide that process of development. This leads to illogical and inconsistent rules in all natural languages.
So, when ever you get aware of an illogic or inconsistent grammar rule in any language, and then ask: »Why is that?« then you know, that it just happened by accident, for no special reason.