What are "custom software companies"?

By "custom software companies" I mean companies that make their money primarily from building custom, one off, bits of software. Example are agencies or middle-ware companies, or contractors/consultants like Redify.

What's the opposite of "custom software companies"?

The oposite of the above business model are companies that focus on long term products, whether they be deployable desktop/mobile apps, or SaaS software.

A sure fire way to build up technical debt:

I work for a company that attempts to focus on a suite of SaaS products. However, due to certain constraints we sometimes end up bending to the will of certain clients and we end building bits of custom software that can only be used for that client.

This is a sure fire way to incur technical debt. Now we have a bit of software to maintain that adds nothing to our core product.

If custom work is a sure fire way to build technical debt, how do agencies handle it?

So that got me thinking. Companies who don't have a core product as the center of their business model, well they're always doing custom software work. How do they cope with the notion of technical debt? How does it not drive them into technical bankruptcy?