Because they learned from netscape's mistake, that's why.
Sometimes full rewrites are necessary and good. The trick is doing it correctly. Rewrites are riskier, more difficult, and require more development resources than greenfield development. More so when you consider that you can't just abandon the old code until the new code is mature and has proven itself.
Most people do rewrites the wrong way, and they get into trouble, but that doesn't mean there isn't a right way. There are many examples of unsuccessful rewrites in history, but also many examples of highly successful ones.
On the contrary, there has been a number of times that I've completely rewritten some code to use a framework or library that was already battle-tested and more fully-featured than the previous effort. I ended up saving myself from writing code that already exists.
At this point oracle can't rewrite Java. If they don't support the current programs, nobody would use it, if they do, they will really be rewriting code that already exists.
The history of the WebKit codebase stretches back as far as Gecko. The earliest KDE HTML work I can definitively establish is in 1997. We got Konquerer in 2000, but Safari didn't emerge until 2003.
5-7 years before something other than IE could again be competitive... Do you really think that disproves Joel's point? I think it may be quite the opposite...