Time is one factor, it's also surprisingly easy to corrupt vfx master files with just a few missing bits.

Checksums will identify something went wrong, and then you need to redownload the file to a quarantine network and scan it. Takes time.

Much easier to go from trusted source to trusted source and verify the files on drive prior to shipping.

Amazon just released a device (forget the name) for exactly the same use case. We developed it in house.

Not to mention most major studios will contractually prevent you from exposing anything to the web.