For a brand name drugs, manufacturing costs are a relatively low portion of the price and margins are high (example: $20/pill, 95% gross margin). For generics, manufacturing costs are a larger portion of the price and margins are lower (example: $2/pill, 50% gross margin).
For generics, because a substantial portion of the price is driven by manufactured costs, margins can be increased substantially by a reduction of the manufactured cost (example: cost $1->0.50, price: $2, margin ->75%, a 50% increase in profitability!). For the brand name drug, the same reduction (example: cost $1->0.50, price: $20, margin ->97%) yields less increase in margin. As a result, there is less incentive for the brand name drug to push for the manufacturing cost reduction (whether or not quality is affected.) And if there is a risk of reduced quality, then the brand name has much more to lose, both in terms of profitability and in the value of it's brand name.
Probably nobody makes their own excipients/binders or capsules. Others contract out API manufacture, and others the whole product.
Shareholders want lowered costs while maintaining revenues, and contracting out manufacturing is one way to do it.
Especially when you revealed all your secrets in your patents anyway.