Because it would be bearing false witness; that is, a lie.
The Roman Mass you want to claim is ancient is not ancient, merely old; it is a codification of the mass used in Rome at the time of trent, after years of slow but steady alterations and acretions; you would barely recognize the mass of 600 or even 800; even the mass of 1250 would be notably different (and that one you can see at least monthly in some areas: the Dominican Mass). And the Mass of 10th Century Spain, still used (rarely) in the Archdiocese of Toledo, is of a different lineage, with notably different prayer texts, even tho they are in latin.
Further, the Gallican and Sarum missals were quite popular in their home regions in 1000-1400… And they are notably different. Not as different as the Byzantine DL, but still different. Over half of the “West” (Europe) did not use the Roman missal. until the 1400’
Oh, and the Missal Text coming forth from Trent hasn’t been used since abut 50 years after… when a pope altered it slightly and repromulgated it. In fact, it was altered about 5 times before V II. nothing world-shaking, but enough that one can tell with an educated listen.