Because it's fun. It's fun stacking it with no cap. Ohh, and it actually works!
Hearsteel synergizes with Teemo:
- Teemo's E base damage is decently high, so he can still deal solid damage while building tank items
- Teemo likes building Liandry, which provides HP for Hearsteel, and its %maxHP burn doesn't care about Teemo's AP
- Teemo likes Grasp in lane, which very much like the Heartsteel's HP
- Teemo's range allows him to proc Heartsteel easier
- Teemo's blind can be considered is nearly a stun against an auto attack based ADC.
Heartsteel covers Teemo's weaknesses, as he is otherwise squishy:
- Malphite Q poke into R combo can kill an AP Teemo
- Assassins are generally a threat to AP Teemo
- Teemo is fairly short ranged, so it can be difficult to use his kit against some teams
- Teemo's Q loses value if he can't get in range of a target that really uses their auto attacks