Well -- TeX is "80s good". We've gotten better at designing ergonomic software since and it really doesn't meet the modern standard. But it's good enough for most people, and sufficiently hard to replace, that it has stuck around.

Added to that, academics specifically are more willing to suffer old crufty stuff than software engineers tend to be. After all their job is to absorb fields of material whether good or bad, and the technology tends to be lagging behind the bleeding edge in many subfields anyway so TeX doesn't even necessarily stand out.