> Can you give me a theoretical example?
I can give you a practical example. I do a lot of photography/videography in my spare time (not professional, have never been paid for it). I shoot RAW photos on a Sony A7RII. Every photo is ~85MB and I can shoot 700 photos in a day. Assuming I shoot one day every weekend, that's 3TB of data a year (that's actually a close estimate when I account for vacations and such too). Then there's video. I own a Panasonic GH5 and record on an Atomos Ninja Inferno, which outputs 4K 10-bit ProRes video at 60fps. This consumes ~1TB per hour and I tend to store it for archival purposes since it only costs ~$25 to do so for pretty much ever.
For a less extreme case, one of my old school teachers has been doing photography as a hobby for ~15 years. She's amassed ~10TB of photos over that time and currently stores them on piles of HDDs and a few spindles of DVDs scattered around her house.
And yes, I absolutely accept that I'm an outlier but any service that offers an unlimited plan is explicitly advertising itself to outliers. They're not putting themselves out there as someone with a 1TB, 2TB, 10TB or even "enough for 99.9% of people!", they're saying that they're giving storage with zero limits.
Your buffet comparison by the way isn't a good one. A buffet isn't truly unlimited, it has a time restriction, a condition that you can only eat what you take, you can't leave with it and the fact that a human being can only physically eat so much. They haven't defined a limit but there certainly is one.