why don't tubed amps have headphone jacks?

Keep in mind that cans are usually 32ohm, so the amp would 'see' much less of a load, putting out approximately 1/4 of the power as it would into an 8ohm speaker.

Normal listening levels for a 90db/meter speaker requires only a few watts, if you quarter that, you really aren't putting much into a set of cans with the volume knob at the same level.