It has to work with the existing C++ code and class hierarchy. And OOP-like API is not good enough.
It was not my assessment though. It was theirs.
[0]: https://github.com/LadybirdBrowser/ladybird/issues/933
[1]: https://github.com/LadybirdBrowser/ladybird/issues?q=is%3Ais...
Not only does it predate the language by a few decades, there is enough type systems variants to chose from.