Two reasons:

1. They don't have the time/patience to deal with amateurs/brides.

2. They don't want to get in the middle of any copyright issues by dealing with images from someone who may not actually own the rights to have them assembled into a book.

I honestly suspect #1 is the prime reason. They price their services with the assumption that they have a very minimal amount of back and forth so they can get the images and design choices in a bundle, do the work and hand it back. They don't have the time budgeted to go back and forth with every random consumer that wants their one album. They'll happily invest the single time effort to train a pro how to submit work to them, knowing that once they've done it they'll be able to get album after album in from the pro in a consistent format and be able to turn it around in a set amount of effort that they built their pricing around. Pros that prove to not meet that expectation will get dropped. Either explicitly and directly, or just by having their work de-prioritized to the point they get the hint and go somewhere else.

(disclaimer, I'm talking from a general feeling in the industry, not about those three in particular.)