Ehhh, systems outside frontend are just as complex. Think of a backend service--you've probably got a low-level TCP/HTTP handler written in a fast systems language like C/C++, then your business logic written in a higher level language like Go/JS/Ruby/etc., and that's interacting with your data layer using a completely different logic-based SQL language (and extra complexity to translate that SQL to your high-level language with an ORM layer), and on top of that you probably have a rules engine for authorization because you realized the systems language was too slow to update and deal with fast-changing business requirements.
This is really not that different from the frontend world where you've got a markup language to define semantics of the view, a logic layer in JS to pull in data with web requests to populate the view, and CSS as a style layer to make the view look good.
This is really not that different from the frontend world where you've got a markup language to define semantics of the view, a logic layer in JS to pull in data with web requests to populate the view, and CSS as a style layer to make the view look good.