Why so many tubes?

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.

@jpwarren58

And I am open to SS to being more complicated.

You do not need to “be open to it”… just pull the lid off one, and like Rev:6 “come and see”.

Why they have so many tubes and why their prices are (excessively) high?

One would think after 117 years of evolution (1904, invention of the vacuum tube) and with only 4 other components involved (R, C, L, transformer) engineers would have reached the optimal configuration for a tube amplifier by now.

But there’s another ’science’ involved. It’s called Marketing ... the art of tempting people to spend top dollar.

^All good there^

There use to be horns and large woofers like 18” (or maybe more) at 16 ohms, and 10-15W tube amps.
Then we got direct radiators, and the work of Theil-n-Small… and later a bunch of low impedance drivers. Like Texton says, “4 Ohm drivers are better”.
But also listening levels have probably also jumped 5-10 dB. I do not recall hearing other peoples music in my youth, unless it was a Ghetto Blaster. Now you hop on most any tram, and every ear bid sounds clear enough to make out the song.