OK does anyone have any idea why conventional "breakfast" food seems to so often come with absolutely no vegetables?

Update on this: I feel like this problem has decreased by ~50% since (my memory of) pre-covid, more and more breakfasts have vegetables, especially in East Asia. Still far from ideal though.

as a vegetable farmer I approve of this cast.

yet I rarely eat veg for breakfast, unless you include some greens in an omelet or with eggs on occasion, or tomato & onion on a bagel with cream cheese

mainly cooked greens are a late fall through early spring breakfast item for me, definitely not in summer.

my typical breakfast is much more protein centric when I'm in season.

I think it depends a lot on your constitution, level of activity, and the season as to whether vegetables are a good way to start the way digestively.

fruits - sugar and carbs do often work into rotation early in the day however.