You would probably change your mind when you learn about API complex infrastructure, a massive number of testing entities that contain hundreds of fields, or the lack of visualized information.
For instance, when QA experts test the UI of an e-commerce website, all information they need usually comes explicitly. If they lack some explanation of what is going on in tests, they may browse a webpage or item description and verify the logic of intuitive attributes like price, size, color, and others.
API testing is quite the other story. It may be hard and puzzling to test fintech APIs, for example, considering the non-transparent nature of financial solutions, security requirements, and strict regulations specific to the banking and finance industry.
QA engineers may have to deal with a wall of text describing specific financial terms, features, or working principles that they need to clarify for themselves before running tests. API testing for fintech may become a nightmare even for a highly qualified QA and testing team.