My understanding is that the Prius uses a CVT transmission.

You can't get added torque from an engine turning at a low speed by placing more load on it to turn a generator to turn an electric motor. If you disconnect the engine from the trans and use it to supply electricity to turn an emotor, your acceleration is then limited to the emotor's modest horsepower.

Always in Top Gear

We have discovered above how the Prius transmission uses an epicyclic gear, a generator and control electronics to adjust the spin rate of the engine without the use of either a step transmission or a conventional CVT. We also discovered that a fixed proportion (about 72%) of the ICE torque is sent mechanically to the wheels. Without the ability to change the gear ratio between the ICE and the wheels, we cannot multiply up the ICE torque to get high acceleration at low speed. Although we've solved the problem of letting the ICE spin at a suitable rate when the car is moving slowly, the torque coupling is equivalent to always being in top gear!

This problem is partly solved already by passing generated electricity from MG1 to MG2 which adds its torque to that of the ICE. Electric motors do not share with an ICE the problem of not generating torque at low speed. In fact, this is where they generate the most torque. If we run the ICE at, say, 2000 r.p.m., with the car barely moving, a lot of power passes from the ICE to MG1. In fact, until the car starts moving, all the power goes to MG1. Even though MG1 gets only 28% of the torque, it gets all of the movement! Since MG2, connected to the wheels, is not rotating very fast, it doesn't take much power to generate its maximum torque. The Prius can launch from a standstill at respectable acceleration up to about 15 k.p.h. using ICE power passing primarily through MG1 and MG2. Only about a fifth of the torque comes directly through the mechanical path during this initial acceleration.