It doesn't but it accumulates cruft and since then new libraries emerge which you might be able to reuse instead of writing your own thing. Just as an example: Boost first appeared in 1999 so very likely at least early on no one used it.
Of course you could do that, but the existence of a bit of code that's survived that long within a project that's been around for 20 years doesn't mean nothing new has happened. Mozilla invented a whole language to make it easier to write browser in; I don't think they won't have considered using Boost or whatever much less radical approach we might come up with here won't have been considered and invested in.
