Many "Index Of" directories haven't been updated since 2016. You might download a 4K labeled file only to discover it is a poorly upscaled 720p rip. Furthermore, many indexes now contain "honeypots"—files planted by anti-piracy firms to log IP addresses.
: A single 4K movie file can range from 14 GB to 32 GB or even higher (up to 100 GB for full Blu-ray rips), making them massive downloads. How Big Are Video Files? - Overcast HQ Index Of 4k Movie