Not just the crt - electrical usage all ends up as heat, so the chips and losses in the transformers, current through the resistors etc all produce heat something like 100/200 watt light bulb. Surround it with a heat insulator and the heat builds up. In my early life I fixed televisions and a constant souce of trouble was covering the top of the sets with fancy cloths. Building monitors/tv's into cabinets with inadequate ventilation is another dangerous and not uncommon practice.

The heater in a crt heats the cathode, which emits a stream of electrons - this is the 'cathode ray' No heater, no electrons are given off so it doesn't reach the front because it doesn't exist.

Stu