Because for people working in those kinds of problems we're talking about (we started from Valve and games IIRC, but it could also be Google, Facebook engineers etc) you don't just download some off the shelf API.
You'll need to create something of your own to:
- have the whole IP
- mold it specifically to the domain logic you need
- take control of it's memory and performance characteristics based on your constraints
- implement it in the language your company uses, for which no ready made A* (or some other algorithm) is available
- you'll need to make it talk to different infrastructure, libs etc
- you might be the one writing the library API yourself
As I said, this is not about what a CRUD programmer will need. But as you can see everyday in HN, people write these and other algorithms all the time in their jobs doing more serious engineering.
In my experience, knowing which prebuilt algorithms and data structures to reach for matters more than being able to implement them myself. Not saying there isn't value in learning or that it doesn't help flex the problem solving muscles, but in most software domains, I think that practically speaking there are more important skills to focus on after you've mastered the basics of time and space complexity.