this is considered to have asymmetrical carbons :ch3ch2ch(chlorine group is connected on bottom)Ch3

Question

why isn't this one: ch3ch2ch (methyl group attach to it) ch3?

By convention, we look at the carbon chains, and carbon sidechains when discussing symettry. If the second were this, CH3CH(CH3)CH(CH3)CH3 we would say it was symettrical.