I was looking at Swain guides and was curious as to why no one seems to build WotA or spirit visage on him. Are these items not good on him?

They are. It's all personal preference. I might get a Hextech early and upgrade it later. Spirit Visage I'm less inclined towards. RoA -> Chalice -> Rabadon's -> Grail has been my preferred core build. If the game continues, I find Zhonya's is more viable to survival and utility than a bit of extra spellvamp. Keep in mind, ALL his skills are DoT/Multi, meaning he's not getting quite as much out of spellvamp as other casters.

Wota doesn't work well with his as his entire kit is Aoe or Dot meaning he only gets 1/3rd spellvamp.

As for spirit visage he's an AP caster not a tank.

In my opinion Swain should be played as a damage soak caster. He falls off late game so building spell vamp with no survivability is pretty pointless. Even just having a Zhonya's wont cut it. I usually build RoA, Rylais, and tank items such as Randuin's Omen, or FoN. This allows you to pull off CC while being in the middle of a fight, and still doing damaged w/o getting focused to oblivion. I would suggest Spirit Visage if you built survivability items with it respectively. Against an AP heavy team a FoN and a SV would work substantially better than WotA and SV. The latter would only cripple your survivability. Swain benefits most in a prolonged fight putting CC on key targets while still putting out decent amounts of damage, and that's all easily done with RoA, Rylais, Abyssal Sceptor, FH, Randuin's Omen, SV, AUG, etc. These builds allow you to get semi-focused by the enemy. I'm not trying to say WotA is a bad item on Swain I'm just trying to state an obvious role Swain can play for his team. The only reason I personally would build WotA on Swain is if 1. My team is AD heavy and needed more AP/Dmg 2. My second to last item for support of an AP Heavy carry ex: Akali/Katarina/Diana