PART of the problem is the extent to which the US industry is wedded to pay differences depending on type, and strict adherence to seniority.
It means in times of movement due to retirement, new types and expansion, as is
the case now, the actual amount of training going on, and subsequent number of pilots in new (to them) types/seats is multiplied as, for example, one retirement of a Captain on a senior type means a Captain from a smaller (but not the smallest) type, another Captain prompted to replace HIM, a senior FO promoted to take HIS place and so on, till finally we hire a newbe FO onto one of the smallest types at the least popular base.
I wonder if anyone has actually done the math on how much US airlines would save in training costs if they paid the same to all pilots regardless of type, as is the case in most European airlines for example, thus dis-incentivising frequent movements between types and bases.
Never going to happen as it is a wholly entrenched system, but it is interesting to contrast what has grown up there compared to other parts of the world.