Why so many tubes?

You need to use a lot of power tubes if you want crank up the output without putting a really heavy burden on each tube.  So that's why you see the 100w tube amps often use an octet for example.  Not my cup at all as I prefer lower output triode amp paired with high efficiency/easy load to drive speakers.  But the technical explanation is simple...to the point this does feel like a trolling thread. 

The speed of your car is not dependent on its cylinder count.

But more importantly there is a general relationship between the number of the cylinders and the output horsepower.  It's actually a good analogy.