It certainly wouldn’t shock me if the answer boils down to “that’s just how it is”, but I couldn’t help wondering whether there might be some intricate historical rationale behind these components each having one more stroke than seems strictly necessary.
For the first, it feels perfectly natural to just keep flowing from the ㇋ into the “underline”, but stroke order prescribes that we lift our pens between the two. How come? Along a similar vein, why are the first two strokes of 廴 (which I appreciate aren’t exactly equal to ㇋) written independently?
I was under the impression that stroke order was meant to maximize “flow”, but that doesn’t seem to be the philosophy at work in these cases? If it’s an ergonomics thing to do with avoiding “sharp turns” or something, I would point to the likes of そ and ゆ as counterexamples.
Many kanji components are simplified forms of other kanji and as as such keep some historical idiosyncrasies in the way they’re drawn. There really isn’t much more to it than that. It’s like asking why we still write the k in knight. Of course when you’re actually writing those characters casually, you’ll tend to just draw them in one as one long stroke.
But this is an art, not a science, it’s not uncommon for the “standard” stroke order to change subtly depending on the character. An example would be 卑 which also appears as a component in 痺 but with a stroke removed:
Note the 11th stroke replacing strokes 4 and 7 of the original kanji.
乃 is also written inconsistently in different characters, sometimes having 2 and sometimes 3 strokes.
The more kanji you’ll learn the more you’ll encounter small inconsistencies like that. This is an organic, living thing, with millennia of baggage and misunderstandings and regional variants.
