That's because people build him wrong, going for really quick AP when you don't have to.
The base damage on Teemo's E is high enough to deal good dmg without having to stack ap, and the Shrooms deal some good damage too (but at lvl 6 you should use them defensively first, offensively later).
Just rush Runaan's Hurricane, since the atack speed works great with the base damage on E and the range of the 2 extra shots is slightly larger than the initial shot (if you play this well, you can poke the enemy champ WHILE farming without having to come close).
After that go for Malady. This further improves your atack speed but also gives the extra magical damage and shred on the MR.
After that go straight up Rabadons (by this time your mushrooms will start to matter more, since you should now be in teamfight phase).
Always go magic pen boots.
Reason why Liandry's is a bad item on teemo:
It only works with his Q and R. Now hear me out on this.
Would you put more points in a real damage item (Nashor's Tooth/Void staff/Zhonya's) or go for the extra 2% health burn on your Q/R (on the chance that someone acually walks into your mushrooms).
The Liandry passive is bad for teemo because he already deals damage over time, there's literally no need to build it.
Teemo is probably one of my favourite champions simply because of the great tactical advantage it gives your team (great split pushing, great defensive positioning).
---------- Post added 2013-05-03 at 08:13 AM ----------