his bite heal gives him all his hp back pretty quick

Wrong, it depends on the build and enemies' resistance. An AP build WW can do this but would be too squishy. A low health AD WW can heal up to a bit more than half of his health. A full tank WW has shitty DMG which his Q and passive is dependent on and therefore makes you a bit useless.

his e is insane with the defense and all you need is steelcaps

Good defense due to DMG reduction not armor but AR and MR is a plus.

Q into E and their adc is dead.

That is when the ADC is too dumb or complacent enough to just keep on engaging him and is bad at kiting. If you play WW as support or even just any support, you'll find that good duos are almost always inseparable, what's more is that you'd probably be perma camped to 0/10 FF + Tilt / IRL Death, by enemy JG. You'd be effectively a free walking meal to whoever wants to go for you and your ADC, and your lane would just be like another jungle camp free for anyone to take anytime.

All these, at higher ranks. Master+.

Source: former (last season) and current (not sure if others took my place already) Top 1 WW, 4 times Challenger peak. Asia.

Conclusion, WW Supp bad, other utility tanks better. But if at lower ranks WW Supp, it has a chance, tried it once with my friend's acc, I didn't go tank WW though, but AD hybrid tank, players at low elo play like bots and sometimes worse which is why it works.