Besides that, have you thought that everyone’s lifestyle may not be just like yours?
But it's more about do enough people want to watch video in public or on the go such that large phones became the dominating choice?
... Why?
Now I am more settled and have a family, I watch things on my phone a lot less.
Rather than using a tablet, I was using a Kindle Paperwhite so that I can take my library with me. I don't use it to browse the web.
I have quite a bit I use my SE, including Evernotes, my meditation timer, my Go client, etc. I don't actually need a tablet.
I have a tablet now. More because my wife and the T-mobile guy convinced me that I can use it as a backup 2FA device with Krypton. There are some use-cases that I am looking to incorporate things with... but again, I don't actually need it. I probably won't take it with me when I travel.
I don't get it myself, but for whatever reason a lot of people do enjoy this. I'm guessing as a coping mechanism. Probably in the same vein as reading books/news, or playing video games etc.
I have an old friend who every night picks out some criterion collection DVD to watch, and posts about it on social media before/during/after.
That said, I have a tablet I use for long-form reading/youtube (90%+ reading) during train/ferry rides > 10 min, but obviously haven't had to use it much recently. Mostly reading.