That's cause WW isn't a traditional AP carry. xD If an AP carry could get as tanky as WW and still do their kind of damage, they'd call him something stupid like the "Master of Metal".
If you want proof, what you can do is look at your post-game stats and check your damage output type. If autoattacks are a big deal, then a grand majority of the damage will be physical. But for Warwick, it's kinda in the middle and it's not rare for his magic damage to exceed his physical damage. This was back before his autoattacks were physical damage of course. Now you'd probably see it skew even more so in the magic damage section, but the point remains. If Warwick was an autoattack type champion, his physical damage should be waaaay higher.
Actually now that I recall... Yeah, I usually went towards CDRwick. The only problem with CDRwick is his mana usage. What was it... like 110 mana for level 1 Q? Or was it somewhere around 90? His Q eats up gobs of mana and now that I think about it, I actually did rely on his autoattacks a lot more just because of the mana cost of Q.
Looking at my most recent game, I actually did more physical damage than magic damage, despite a Wriggle's and Wit's End. Although, they also surrendered early thanks to an awesome Swain and some most profitable ganks. So maybe it didn't reach the lategame, where 20% of max health becomes heavy damage.
Also, his ultimate does mostly physical damage, with regards to the magic damage from his passive.
Actually, I still do mostly physical damage, even though I tend to rush Wriggle's and Wit's End.
EDIT: Actually, come to think of it. I've played bot games where I've built on-hit/spell-vamp with Wit's End/Malady/Gunblade and I still did only 3000 more magic damage than physical.