Why there is a low frequency?
Everything looks right about your calculation, AND your plot! Nothing's wrong; there's no other frequency than \$f=1\$ in here.

What is confusing you is the fact that your excel plot incorrectly connects the sample points with straight lines. That isn't how reconstruction works!

You'd need to sinc-interpolation (with infinitely long sinc functions, that you shift to every sample position) to get the original sampled signal back.