Similarly to many other open-source design apps, KiCad has a fairly horrid UX and lacks some enterprise features (especially around collaboration on larger projects), but it's definitely being used for complex projects at a good number of companies.

And the reasons why professionals and organizations are paying huge amounts for specific software are often less enlightened than we tend to assume. Inertia is a huge factor. Many universities train students to use Altium, for example, so it's often the path of least resistance. Plus, when you have processes and teams built around a tool that was cutting-edge a decade or two ago, it can be hard to switch. Especially for niche software like this, interoperability tents to be poor, every vendor invents their own UI and terminology from first principles, etc.

But KiCad and Altium are not the only choices. There are affordable "serious hobbyist" products with much better UX (e.g., Diptrace) and there are higher-end commercial tools that leave Altium in the dust.