Why is bandwidth always limited in a real (physical) channel?
Information bandwidth is dependent on signal to noise ratios.  At absolute zero, quantum level signal quantization and quantum noise will limit the lower bound on the noise floor.  At higher temperatures, thermal noise creates a higher noise floor in any information receiving equipment.  There may also be an upper limit on power density in a signal before its mass equivalent creates a black hole, which may be problematical in terms of information conservation.  In terms of transmitters, at some point you will melt the wires.

But in terms of pure math, there is no (aleph zero) finite limit on the amount of information that a single real number (one sample) can contain.