Because following the plain and clear text of the clause is just too complicated. Better to just accept what everyone says.
You are very vague on the concept of a slave arriving in a freestate as a fugitive and a slave being brought into a freestate accompanied by his master.
I am not vague on it at all. I note the clause says that the slave will be returned. I see no qualifications that says "under certain conditions."
It is you who somehow imagine that there are conditions under which the law means something different.
And you are equally as welcome to show me where it doesn't allow that.
I have done so a half dozen times or more. It says so in the very clause. You just don't want to accept the ugly truth of what the plain text of the law says.
In fact, as HandyDandy & others have point out, and you obtusely refuse to acknowledge, the Constitution refers specifically and only to "escaping" slaves, and places no limits on states-rights to declare legally transported slaves freed after certain time limits.
That understanding was acknowledged and obeyed by President Washington in Philadelphia, so Founders' Original Intent in the matter is utterly clear, regardless of Roger Taney's lunacies.