To get bigger, stronger, faster, we use resistance to work our muscles to exhaustion repeatedly. This is what strength training is for.
To become more flexible, we stretch to lengthen our tendons and muscles and increase our range of motion. This is what yoga is for.
In tai chi we don’t do either of these things, so what is tai chi for?
Tai chi is for building the nervous system. In tai chi you don’t see the results as bigger muscles or increased range of motion, but rather as the effects of increased neurological development. Through tai chi training you become more receptive and responsive to internal and external feedback. Physically the aim is to increase effectiveness, rather than capacity.
In truth all three of these types of training (strength, flexibility, sensitivity) have distinct and complementary benefits, so a balanced physical training regimen will incorporate all of them, regardless of what it is called.