The "Just" is doing a lot of heavy lifting there.

Think about the presenting data part of it. Perhaps you have a table of data, prices of tickets or some such. You could literally just wrap each item in a <td> tag and each row in a <tr> tag and that would indeed be easy.

But people generally want and expect more. How about color coding the rows to make them easier to scan left to right or sorting the table by each column or paginating through more data than can easily displayed on one screen or maybe you're doing "infinite scroll" instead.

None of these things are impossible of course, and people have done them so many times and in so many different ways that there are dozens of libraries you can use and hundreds of tutorials, but even so, compare that to the "select from table turn to json" equivalent.

The SQL can be more than a bit tricky, but aside from that, JSON is extremely well defined and specified, even without a library you can just read the specs and do it. Wrapping it in a HTTP response and returning it and so on is likewise very well specified and if you can read, you can follow the instructions on how to do it.

Creating a UI that works the way a user wants to is the opposite of all that.

Of course, one of the major differences here is that at any point you can just stop improving the UI. Maybe you stop after wrapping it in the table html. The UI will certainly work, for a very specific definition of work. JSON is considerably more boolean. It either is a valid JSON document or it isn't. You can ask a computer to check for you. You can't ask a computer to check if your users enjoy using your table.