I can think of several reasons:

1. You are primed to think that it is mysterious because that’s all you usually hear about assembly. (“Roller Coaster Tycoon was written in 100% hand crafted assembly… what an absolute wizard!!”)

2. The language’s textual format is odd - columns vs nested indentation. Actually really nice once you get used to it, but it’s definitely alien at first.

3. Mnemonics and directives have short, cryptic spellings. x86 in particular has arbitrary looking register names as well. RV, AArch64, m68k etc do better here.

4. Mnemonics are inconsistently overloaded and encode lots of stuff. SIMD instructions tend to look like a cat sat on your keyboard.

5. Manually laying out memory is technically simpler than the abstractions provided by higher level languages (structs and classes, fancy generic types, pointer syntax), but it’s fiddly and you have to deal with alignment.

6. You have to do a lot of bookkeeping yourself. It’s like malloc/free turned to 11.

7. Register allocation is a hard problem for computers. It’s kinda tough for humans, too.

8. Lots of books and online stuff discuss assembly for use with high performance code, tight compute kernels, raw hardware access, and fiddly CPU configuration for OS startup and virtual memory configuration. This requires even more specialized registers, arcane instructions, and bit fiddling. This stuff - along with reverse engineering and security research/attacks - gets lumped into what people think of as “assembly language”. The resulting concept surface therefore looks much larger than it actually is.

I highly recommend making a non-trivial program entirely in assembly at least once. I need to do it occasionally professionally but even when I don’t I usually have a hobby project or two cooking at home.

Becoming as proficient in asm as - say - C or Python is quite the lovely expression of craft. You feel like a wizard (see point 1) while simultaneously learning what’s really going on.

For people with a certain geeky disposition it pays lots of aesthetic, psychological, and professional dividends.