There is literally no competition for the 16.3-inch MacBook, which comes with 1600 nits brightness, or something like the LG Gram laptop with a 17-inch screen, but it only has 350 nits brightness.
Why don’t we have a laptop with good battery life, a 17-inch display, and high brightness?
Thats a good question, but I doubt that someone here will be able to answer that question.
But I agree, that I would really like a display with even more nits for working outside…
The 1600 number is peak brightness. MacBook Pros mostly aim for 600 nits max (with “SDR” content), with 1000 nits of full screen brightness when displaying “XDR” content (XDR being… Apple’s name for HDR? 
). They also only support the higher brightnesses up to 25 degrees C. MacBook Pro - Tech Specs - Apple
Apple’s big and rich. They’ve been pushing the limits of what can be manufactured at scale for a while now (for laptops that basically cost double). Everybody else will gradually catch up over time.
Framework could potentially offer a brighter display in the future if they want to. The Framework Laptop 16 uses the BOE NE160QDM-NZ6 display.
BOE (who supplies all the displays that Framework uses) also makes the related NE160QDM-NM1 and NE160QDM-NM4 displays which are rated at 1250 nits.
Those do have the controller board on the back of the display, which makes them too thick to put in the Framework Laptop 16’s lid (the NE160QDM-NZ6 that Framework uses has the controller board in the bottom bezel to solve that), however the NE160QDM-NZ6 is already a customized variant of a different BOE display so Framework may be able to get a customized variant that would fit.
You actually need the peaks to be able to see something in full sunlight on a glare display.
TBH, I used my FW16 last week in full sunlight (south of Spain while diving in l’Estartit) and was pleased with how well my screen was readable and usable.
A dive-buddy of mine using a mac-book couldn’t use it. Too many reflections etc. Had to turn around out of the light to be able to actually see something. Think I converted him to Framework… He asked me to get the details for one of these beasts 