The Makedonian phalanx -- why such depth?

[PMBardunias]

We know of one kind of oblong shield being in use in Thrace perhaps three quarters of a century before the Galatian invasion, while from the east the only oblong shields I can think of being in use around that time is the gerrha of the Persians.

Those Thracian shields are what I was thiking of in particular. I have opined previously that Iphicrates famous reform might have been a Proto-thureophoroi rather than a Proto-sarissaphoroi since a 12' non-tapered spear can be used with one hand and has roughly the reach (6') of an 8' counterweighted doru held at one third from the sauroter (5.3'), but is nothing like a sarissa. That whole "symmetrical" shield thing could be describing an oval thracian shield. Also, I have yet to find a description of the Egyptian shields that Xenophon mentions you can push against with your shoulder, but these seem likely to be oval- perhaps oval on top and squared at the bottom as in earlier shields form egypt.