The second strength of models actually is their variation. For not all of the variability of internal models is necessarily undesired. There may be good reasons for divergent capital requirements based on similar credit portfolios, for instance because of dissimilar effectiveness of risk management in banks or given a different legal environment in which banks are operating.
There is a clear incentive for market participants to have less regulation, at least for the part of industry in which they operate. There is thus a strong incentive to lobby the political sphere to reduce legislation and regulation, in a somewhat vicious circle.