Here's why:
* Frenzy Plant is a move exclusive to the final evolution of the Grass starter in the Generation VI games (X and Y). This means it's not available in SoulSilver.
* Meganium is the final evolution of the Grass starter in Generation II (Gold, Silver, and Crystal). It learns a variety of Grass-type moves, but Frenzy Plant is not one of them.
If you're hoping to have a Meganium with Frenzy Plant, you'd need to transfer it from a Generation VI or later game.