I thought the same until last year when I had these awkward 3 week business trips to our various satellite offices.
I would bring my small laptop, set it at my temporary cubicle, and basically leave it there for the duration. I always asked for a spare monitor, but sometimes there wouldn’t be any at the smaller offices, or they would be crappy TN monitors.
It was then that I realized having a large screen would be very convulsions.
I find I use the 15" laptop screen more when it's on my desk than the smaller laptop screens. Sometimes you want to get work done so ignoring other distractions on secondary screens are great. i can go full screen at 1680P and not feel out of sorts but full screen on a larger display doesn't do it for me.
I usually find a coffee shop with large enough tables intended for laptops. I just enjoy being out and doing work on my 15” screen from time to time.
You can pack more battery into a larger body.
Speaking for myself, 15” is a comfortable size to write code on using a larger font, due to slight vision issues.
If I'm using my laptop on the go, I prefer a small screen so it's usable on a small plane tray, on a cramped cafe table, in a conference room where I don't want my device to distract, etc. And weighs as little as possible.
While if I'm using my laptop for real work, at work or at home, I want to plug it into an actual large screen, which 15" or 16" is not -- it's just too small. Let's do 25" or 30".
Large laptop screens are this weird middle ground for me that seem to be worst-of-both-worlds.
What am I missing?