For a small site maybe, but you underestimate the challenge of some of these things for enterprise sites. If a new directory or slug is added to a site, what team in an organization is going to care if all urls are redirected correctly? For a site with tens of millions of pages, who is going to make sure they're all working the way we want? Who makes sure robots.txt files take into account those changes? Are sitemaps updated for crawlers? Are all our analytics tools in line with the new url structure? etc..

Every large site I've worked on doesn't actively think about these problems, and it's the SEO team who has thought about these problems.

Generally people care about their slice of responsibility. At most large organizations with enterprise sites, many parts of SEO falls outside of those slices of responsibility so people don't think about them. This is especially true if it's hard to put dollar values on them.