If you build a robot that only handles the main workflow and can't handle any of the exceptions, you're not automating anything; you're building a convenience tool. Basically, you're removing some of the labor part of the job, but still keeping the decision making part of the job for humans.
This is both economically unviable and largely unpopular. You're not reducing labor costs by much, because you still need humans around. And workers lose control over their job, and have to "work around" the robot's limitations. Instead of being freed up to do more work, they become babysitters of machines that they have to oversee so it doesn't mess up everything.
This is why partial robotic systems don't really exist; either it's a nifty tool that speeds up a small repeatable process of your work, or it's building the entire house.