Did you know that Americans and English have different names for vacuum tubes? What Americans call "Tube," the English call "Valve."
This may seem a bit odd to us Americans at first, but while "vacuum tube" refers to just the physical nature of the device, the English are referring to what it does.
A vacuum tube conducts electricity by creating a stream of electrons in a vacuum. To do useful things with this (like amplifying signals), we have to be able to control this stream, to make the flow higher or lower.
You can think of this as a valve controlling a stream of water running through a pipe. Water (current) runs from high-pressure end to low-pressure end, and you control this like a valve.
Hence the name "Valve."
So now that you understand how a tube works, the name "valve" kind of sticks, doesn't it? Leave it to English to be precise about the names of things, but in this case, we think the English have it right.