Because some of us travel?
"isn't it better to buy a 13-inch tablet then?" No. Tablets are for consuming media and are terrible at anything else. Plus with a keyboard case they weigh more than at 13" laptop.
They are not necessarily great for consuming media, either: in my experience, tablet screens get mucky very quickly, not least because a tablet's primary mode of interaction is rubbing your fingers on it. I think tablets are nice devices, but I wouldn't buy one in preference to a laptop unless I had a specific use-case in mind.
I think the size and proportions are ideal for a small, thin laptop. Conversely all of the large tablets I've tried have had uncomfortable detachable keyboards and OK at best touchpads.
13” tables are just weird uncomfortable laptops that lack a keyboard.
I assume they're taking the same approach as Apple, and have the same input components but just padd around on the larger models.
The screen resolution is a tad too low on the framework laptops, but 3~4K would allow for enough pixels to reduce screen elements comfortably. At lrast that worked pretty well with the macbook pro, 13" at max resolution was good for 99% of my work.