You mean a modern construction with semiconductor-based SMPS, but tube for output? If so, plenty of those, here is hit #2 in my google search for "guitar tube amplifier": "Orange Micro Terror" [0]. This one take 15V DC input - no way you can get this to tubes without some sort of SMPS.

Or do you mean why people who do period-authentic tube amps don't use SMPS? That's because tube-based SMPS is very complex, often as complex as amplifier itself, and needs unusual parts [1].

[0] https://www.sweetwater.com/store/detail/MicroTerror--orange-...

[1] https://www.righto.com/2018/09/glowing-mercury-thyratrons-in...