Why so many tubes?

More expensive preamps may use different tube configurations like cascode, paralleled, constant current sources, regulation etc.

Not sure what is meant by 'direct path'... But if you need a lot of power, you might need a lot of tubes to make that power. If so, they'll be in parallel, which means the signal is fed to all of them and they all work in tandem to make that power. In that particular case, the signal path complexity is the same whether one power tube is used or 20. 

But… the signal is added up coherently, whereas the noise is adds non coherently. So in theory it is better to have the elements in parallel.

I live in a cold climate and need the heat that the KT150's provide.  4 per amp and my sound room is comfy from October to May.  During summer... well, that's another story.

Atmasphere nailed one good reason (duh).
There are quite a few others, from using tubes to rectify, regulate, as differential amplifiers....none of which greatly complicate the signal path.  Plus several gain stages, each very linear, are sometimes better than one gain, over stressed stage.
And a million more answers that really benefit from a fairly extensive understanding of electrical engineering....

Bob Carver's Silver 7s run 20 KT120s per mono block!