[dmix]

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

[teraflop]

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.