Using the bitcoin client to have a pruned version of the blockchain can take a lot of time, I was wondering what would be the drawback to simply download an already pruned bitcoin blockchain file from a reliable source like bitcoin.org then put it in my bitcoin directory and then use/update it from it there.
It introduces too much trust into the download source. It's the same thing as just uploading a datadir. You aren't verifying any of the blockchain history; what you download could be a fork of the blockchain. Having such a download completely defeats the point of running a full node.
Furthermore, with downloading a pruned datadir, you will have to download the chainstate. This isn't even verifiable without downloading the blockchain itself. The uploader could have modified the chainstate to include UTXOs that don't exist, and your node wouldn't know that. You could be forked onto a separate chain or just contain invalid things in the UTXO set.
At least with a full blockchain datadir download, you could ignore the given chainstate and just verify the blocks. But that's equivalent to the old bootstrap.dat download, and isn't actually faster.