Multiple reasons.
First, the Constitution (this applies to statutes as well, if not more so) is not the product of a single intent. It is the work of many people, each with their own intentions, desires and opinions - often at odds with each other. Constitutional, as well as statutory language is the language of compromise between differing intentions.
Second, we don’t have a time machine - we can’t go back and ask them what they meant. You can attempt to divine the intention by reading debates and discussions - but that just becomes a battle of selective quoting. An example of this is apparent in the birthright citizenship thread - where multiple posters on both sides of the issue have quoted the same Senator to argue for their side.
