Ah, that's odd then.. Maybe your video card/drivers don't support it? (Though that'd be strange..)
You could try bypassing the options and just put it in the config file and see how that works out? Just do back your settings up before doing that, maybe your pc really doesn't like it.
Go to your ..\World of Warcraft\WTF\ folder and find the file called "config.wtf". Open it in something like notepad and find the line that says:
SET gxWindow "0"
and change it to:
SET gxWindow "1"
It probably already is on "1" since you're playing in Windowed Mode, in that case you can leave it as that.
Next we can adapt it to enable the Maximized thing by finding the line with:
SET gxMaximize "0"
and change it to:
SET gxMaximize "1"
If that line is not in the file, add it under the gxWindow line.
Save the file and try again.
I found this bit of information in the following thread, it's not entirely about the same thing, but the settings still do the same thing.
WoW-Europe.com Forums -> Windowed Mode
As for the "Maximizer" thing suggested by Kulaar. It seems that it removes the borders and titlebar, but not the Windows Taskbar. It could be a setting though, didn't see anything about that.
I hope this works, because I can't find much by Blizzard about missing Video Options.