Why must they be kept so cold?

These machines are kept so cold to reduce heat and noise that cause errors. Superconducting qubits rely on maintaining an extremely delicate quantum state, and any stray thermal energy introduces random vibrations and electrical noise that can knock a qubit out of that state, an event called decoherence. Cooling to a temperature colder than deep space strips away nearly all ambient thermal energy, giving qubits a better chance of holding their state long enough to complete a calculation.

The extreme cold has nothing to do with making qubits glow or with internet speed; those explanations describe effects unrelated to how superconducting circuits actually function or fail.

The dilution refrigerators used to reach these temperatures work in stages, and the coldest inner stage where the quantum chip sits is colder than the vacuum of outer space, which still contains faint background radiation left over from the early universe.