Solr needs opinions, because Solr documentation gives you way too many options. It's hard to navigate the best practices for using Solr. Some of my opinions based on dozens of Solr projects :)
Schema files are a good thing. They are declarative, and not letting them change at runtime prevents all kinds of security issues. Further, classic schema / solrconfig support all of Solr's functionality and are well documented with tons of examples online in blog articles and stackoverflow. Using managed schema or the config API takes a lot of experimentation.
Static configurations can also be easily version controlled. As I've learned as a long time Elasticsearch user, this is one of Solr's advantages. Having an API for changing every underlying config option of your index means finding the code that made the change is rather time consuming.
Static configuration is also good separation of concerns. You cleanly separate