I gotta think it's because all the math you mention cannot render within a normal line height. That's not the end of the world for rendering, but maybe that counts as a bridge too far in Unicode? Or maybe it just boils down to "because it doesn't yet" and as soon as someone makes a good proposal it'll happen
Those are a ton of combining accents put together. Crucially: they don't make the line any taller—that's what makes them so hard to read! An integral should be a little taller than the line height but you still want the integral's limits to be legible. Maybe I'm being too prescriptive; ∫₀¹𝑥 𝑑𝑥 would read ok if that 1 were stacked on top of the 0 without any other changes.
My apologies if HN's formatter strips the special characters (edit: it didn't yay!): I wrote integral sign, subscript 0, superscript 1, math italic x, thin space, math italic d, math italic x. And maybe the idea is we should use words like "the integral from zero to one of x with respect to x." IDK.
