In the case of Ladybird they build everything from scratch (intentionally) so existing open source code cannot be used.
The main argument for doing it is because it is fun, just as with SerenityOS. Having alternative implementations is never a bad thing for web diversity though.
Of course, alternatives are almost never a bad thing, and devs should feel free to work on whatever floats their boat when they're volunteers, but developer resources are scarce, and there are other things in the FOSS realm that could use some attention where there really aren't great alternatives. But again, if these people prefer to work on this, that's OK. Just because the world could use a better X doesn't mean these devs have enough interest in X to be effective at building such a thing in a volunteer capacity: in my experience, having personal interest in a project makes you much more productive than working on something you really don't care about.
