Yes, WordPress is a tool with an old architecture. WordPress doesn't use all the hype design patterns but WordPress is a simple software to make websites with a huge community.
Of course, WordPress core developers could rewrite the code to have a more modern conception but what about all existing plugins, tools, etc ? I think it wouldn't be a great idea.
When I develop a plugin for WordPress, I know that I could help a lot of users. I can code using pure PHP code and WordPress doesn't force me to use specifics things.
I just have to focus on the problem I want to solve.
(Post on 2018-06-22)