probably because the vacuum of space is way more extreme than inside the tube which would cause pressure on the seals

Vacuum tubes typically have an internal pressure of less than 0.001 atm, sometimes much less.

Any seal that can withstand a pressure difference of 0.001 atm to 1 atm from the outside can almost certainly withstand a difference of 0.001 atm to 0 atm from the inside.

Possibly nowhere for the heat to go. Some tubes need a certain amount of ventilation depending on the application. It wouldn't pop, but might fail.

Some of Soviet spacecrafts were known to have been built around ~1atm pressure vessels as a brute force means to reduce unknown unknowns. I suspect it could be reverse reasoning from there.