why don't tubed amps have headphone jacks?

I can understand that vintage amps usually didn't because the state of phones in the 50's was rather primitive and phones weren't useful 'til stereo came in strongly. Why no phones jacks now?
You mean tube preamps, Not amps??? Many have it, almost I would go as far to say 80% of the pres and intergrateds have them now.
Because there is some really good, dedicated, phones amps out there... MHO.

Amps? I've never seen a headphone jack on an amp, tubed or solid state. In fact, you physically can't have one on an amplifier without adding a volume pot and an entirely seperate apmlification section (else you'd blow the cans). And if you did that, you'd have an integrated amplifier (with both pre and amp sections), not an amplifier. You want an integrated amplifier, fine, but that's a seperate beast from a straight amplifier.

As for headphone jacks on tubed integrated gear (or preamps), sure, seen it plenty. Generally, it's added cost and circuitry that many purists will prefer to exclude -- if you want a preamp, buy a preamp, if you want a headphone amp, get one of those. As price and quality go up, simplicity is typically a driving force as well. By doing fewer things, you can do those things better. But, folks definitely do it. I've had two tubed preamps (VTL and Rogue). The Rogue (99 Magnum) has a headphone jack. A pretty nice one, actually.

Does that answer your question at all?
Mezmo, you don't need a separate (opamp) amplification circuit. The Cary 300SEI (integrated) has their headphone jack wired directly to the output transformers. That's why this particular amp is so coveted by headphone devotees.
My Audio Space AS-3i integrated has a headphone jack. A nice one, too, not just an afterthought, and that's one thing that tipped the the scales in its favour.
The Cary SLI-80 integrated has a very nice headphone amp.
I purchased a Cary Audio SLI 80 F1 for a second system. The main reason was it has a really nice headphone section and jack.
Pawlowski, cool. That said, I had intended to mean nothing more than you wouldn't want to drive cans off of speaker taps (or the final amplification bits of an amplifier intended to make speakers go). Can't imagine that cans on 250 + watts would fair so well. Thus, however you get it done, I presume you'd need more than what you find in a vanilla speaker amplifier. But, clearly, I know just boatloads about "electic bits...."
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.
My Tenor 8.5's have two jacks
Heh, look at me all learnin' and stuff.