I guess the context in which I'm wondering about this is that some problems are being solved over and over again, rewritten in the language of the moment, and so on. Like, GUI toolkits are also insanely hard, yet there are like 5 WIP ones in Rust.
Browser engines are maybe similar in that there are basically no from-scratch implementations, but there we have embeddable libraries you can build an app around. The same doesn't seem to be true for office apps.