I mean that it’s not a technical constraint or limit. Not that its trivial to do! If there is an ROI in keeping stuff forever, you can.
Think about what you’re saying about your personal project. What you’re talking about wasn’t financially achievable for TV companies just 20 years ago. Hell, anything older than a few days would be stored offline in some tape silo, or in a shelf. (And 10-12 people to attend to it!) Now you as an individual could afford to do all sorts of stuff with that video data, albeit for some significant $.
Professionaly I work on a financial reporting system where there are around 1.5 million accounts with trading activity. Things like moving a month of data around and checksumming still take on the order of 1 hour. If the size of that monthly dataset doubled, we'd be OKAY but it it grew 10x we'd have to start restructuring things both on the software side and the hardware side.
For personal work I am working with searching video frame substrings regardless of codec, resolution etc within videos in a ~50 TiB video set. Having to re-encode hundrends of GiB of video at a time is some thing that takes a huge amount of time and nothing something I can just "throw into an S3 bucket". The growth of said working set is not truly constrained but is actually constrained. I can only afford so many 8-10 TiB HDDs at a time! They also fail and have to be mirrored/backedup with rsycn at the very least which adds to the cost for me.