WordPress uses an outdated approach to coding
Procedural programming is generally easier to learn than modern programming methods, but is less scalable, secure & stable. Most modern software & professional developers now use more structured methods like object oriented programming for core functionality.
The problem with Wordpress is that it restricts the Designer. As developers, we have the capability to build anything the client would like bespoke, but sometimes Wordpress doesn't allow us to implement that. So Designers can get frustrated because their ideas are not possible within Wordpress.