Why so many tubes?

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!  

Was this a troll thread?

Who cares the platform as long as it produces high quality reproduction of music?

I thought that it was like 💰 money. The more the better!😄

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.