Mainly because no browser is really best for everything.
IE == definitive Windows standard, but doesn't exist on *NIX
Netscape 4.x == absolute crap, lousy fonts, crashes a lot, but handles more in *NIX than any other browser.
Netscape 6 == worthless. I don't install it.
Mozilla == best for *NIX, but horrible startup time. Bit of a memory hog. Also can't load the few sites that are hard-coded to reject "Netscape 6" browser IDs.
Konqueror == Nice, lightweight browser, loads very fast, but gets confused on a few pages. Also no really good Java engine available (it doesn't come with one by default). *NIX only, I believe.
Opera == haven't tried it in a while; don't like paying or adware.
bash-2.04$ kill -9 1
init: Just what do you think you're doing, Dave?