In college you're pretty much always working in a greenfield (everything is new and you don't have to worry about any legacy). In a job, you'll almost always be in a brownfield (you have to work within the existing systems and legacy to make things work).

There could have been decisions made 30 years ago when the compute landscape looked very different that are still impacting decisions today. Many times you might think something is stupid, and the person telling you about it probably thinks it's stupid too, but it's what needs to happen to keep things working without upending the whole company. The person you can both yell at about it is probably retired or dead, and likely made the right decision at the time, given what they were working with at the time.

Even with projects I've had that are relatively greenfield, eventually they have to meet the existing infrastructure. When that happens, my perfect little applications has to get hacked to bits to cover various edge cases due to a company acquisition or some section of the environment that is non-standard for whatever reason. I find this part of the job quite painful, but it's necessary for the tools to actually work.

We've brought in some vendors from time to time who are supposed to write some code for us. They follow best practices and do everything by the book. There stuff doesn't work. We end up having to throw all of it out and re-write it the second they leave. Production code is much more ugly than anyone wants it to be.