Many plugins add dynamic pages to WordPress - e-commerce carts, membership plugins etc. If WP would just add a cache to everything they would not work. You migh

Many plugins add dynamic pages to WordPress - e-commerce carts, membership plugins etc. If WP would just add a cache to everything they would not work. You might say that there could be an API for these plugin to register their uncached routes, but not everyone runs Apache - Nginx is also commonly used and would also need to somehow be supported. As someone who has written their own WP caching plugin, it's actually very convenient to perform the caching in PHP, because you can check for cookies and regex expressions for URLs, and there are many plugins that handle it well. For most sites I use a free one called Cache Enabler.