Manufacturers have longer QA cycles and management dictates where resources go.
The major manufacturers that will be releasing updates this month had them written months ago.
A custom rom developer can test with community help, release more quickly and no one will scream and rant and return their phones, or post all over the Internet not to buy phone brand XYZ if something in the rom is broken.
People are generally understanding of custom rom errors.
Not so with manufacturer updates.
As for proprietary drivers - sometimes those are able to be pulled as binaries, no source required - and more often, simply can't so the custom rom loses the associated feature. (Good luck finding a CM rom that has MHL output, or an AOSP rom that can update your carrier credentials or even GPS settings in many cases. I could go on with examples.)
