T1 signal is the output of a 24-to-1 digital multiplexer whose inputs are 64 kbps each (plus one framing bit).
But a multiplexer is simply an electronic device that selects between several inputs and connects it to a single output. Therefore the line rate of the output must be the line rate of which input it is connected to.
So if each input could only transmit at 64 kbps, how could the output line be at 1.544 Mbps? How could the multiplexer transmit at a rate higher than what could the input possibly do?