Why does DTFT start and end at the same magnitude?
DTFT is a periodic waveform;

$$H(e^{j \omega}) = H(e^{j (\omega + 2 \pi k)}).$$

Hence for every frequency interval $[\omega_1, \omega_2]$ of length $2\pi$ it will repeat itself; i.e., beginning and ending at the same magnitude (and phase) on that interval.