Can you cover tubes?
It is for heat dissipation.  And tubes get hot due to the current flowing through them.  It's the work they do that makes them hot, rather than that pretty glowing filament as you might expect.
Having said that, look up tube shield. These are metal cans that fit over the tube. They are used as electrostatic shields in the same way as shielding over other electronic components.  These shields usually are in contact with the glass so they can absorb the heat and radiate it themselves.