Sure, but what heuristic are Google going to use to detect encrypted files? What if that heuristic matches files which aren’t encrypted and starts deleting those? We’d start to see cases of “Google deleted all of my RAW photos”, or whatever special binary format Google haven’t coded for.
Clearly they have a heuristic for identifying pornographic videos that they have enough confidence in to use on what very well might be irreplaceable footage that is not pornographic and/or does not violate ToS. I can assume that they can find something similar to identify encrypted content, perhaps simply by identifying files of unknown format with high entropy. Short of steganography, which is inefficient, I don't think you can prevent Google from knowing you're storing encrypted files.