Ask HN: Why do so many sites keep breaking their page URLs?

Seriously, it's 2017, and 301 redirects are a thing that exist. So why do so many sites still keep breaking all their page URLs? It's not like its a CMS limitation; most modern CMSs automatically redirect pages to their new URLs. It could be developer incompetence or laziness, but many sites that make this mistake are run by large companies with hundreds of employees, some of which are likely really good at their jobs. So why do sites do this? Why are 301 redirects seemingly so rare when sites are restructured nowadays? Because it's really hurting the usability of the internet, and making forums and wikis far harder to use than they need to be.