The Makedonian phalanx -- why such depth?

[Paralus]

2. We are NEVER told that Hoplites actually fought "8 deep" or "12 deep" etc - Xenophon speaks of 'battle formation' 4 deep....

And he is the only one who mentions it – and only in that context:

Anabasis, 1.2.15:
He ordered the Greeks to form their lines and take their positions just as they were accustomed to do for battle, each general marshalling his own men. So they formed the line four deep

He also mentions what every other source regularly describes:

Anabasis, 7.1.22-23:
Then he proceeded to pass along this order himself and bade the others send it on--to ground their arms in battle line. The men acted as their own marshals, and within a short time the hoplites had fallen into line eight deep and the peltasts had got into position on either wing.