bluray quality video is about 13GB/hr for 1080p or 33GB/hr for 4K. it's cost prohibitive for people to accumulate a huge amount of this content legally, but torrents are still a thing. it adds up really fast if you aren't willing to prune your collection.
there isn't currently a ton of 4K content available, but there's lots of 1080p. if you rip/download enough bluray 1080p to watch an hour every night, that's almost 5TB per year right there. if there were enough 4K content to support your habit, it would be more than 12TB a year.
Assume average movie length of 90 minutes and 4k Blu-ray price of $20 (I don't buy physical Blu-ray's but found some year or two old recent marvel movies cost about that much - new releases cost more, typical boxsets come out as costing less per item)
So that's 50GB/20$. Or $400/TB. So the content to fill those hard drives is about 5x the drives yourself. So it's not infeasible that people could fill them with legal movies.
Not to mention TV shows, video games, and content that is much denser in GB/$.
When my kids were young and could ruin a DVD just by taking it out of the case, I put together a home theater PC and ripped all the kids DVDs to it. There's well over 100 movies on that disk, and I still have the original DVDs for backup.
there isn't currently a ton of 4K content available, but there's lots of 1080p. if you rip/download enough bluray 1080p to watch an hour every night, that's almost 5TB per year right there. if there were enough 4K content to support your habit, it would be more than 12TB a year.