When meteorites fall on Earth and you pick them up, you are able to date the time of their formation.
You say: You have to distinguish the time that the meteorites form and the time that they hit the Earth.
Most rocks on Earth have melted time and again and thus are useless for figuring out how old the Earth is.
Simple enough, see the meteorites formed alongside the planet, however, since the planet was pretty much a molten soup you can't date it properly, because we can only date it after it cooled down.
In light of the dramatically plastic and ephemeral status of published ages, the investigators were wise to express a measure of insecurity regarding these “oldest, most brutally battered terrains”: Considering what would have happened to the earth’s crust if it really was shattered, re-formed, and then warped by a cataclysmic, globe-inundating flood as the Bible and other ancient documents testify, it is entirely justifiable to reject these billion-year dates.
Actually the crust cooled rather quickly after the formation of the Earth.
Not that it matters since everything was blown up again in the moon forming event.
Short answer: because the meteorites formed together with the Earth and the rest of the Solar System.
Long answer: The Earth formed together with the rest of the Solar System and its meteorites around 4.5 billion years ago.