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.

My McIntosh 2505 has a headphone jack.

Trucker/Mezmo; one of my favorite pair of headphones was the AKG K1000. It didn't even have the normal 1/4" headphone jack. It connected directly to the speaker terminals. People have driven them with all type of amps!

I have a Gini Audio Space 6i intergrated tube amp.
It has a really nice headphone output and it can play loud.
It uses KT88 output tubes and amp is really built.
Here's a pic of it.
http://www.gini.com/index.php?id=int-amp

My Tenor 8.5's have two jacks