The problem is there is trivial and then there is trivial. Yeah you can get a notepad clone up and going quickly. But if you want to compete with other editors that are out there for free you need to add syntax highlighting, efficient support for large files, likely tabbed content support, fonts, good search, potentially regex support etc. In short: we rarely need to create something completely new now. But we have an endless list of required features to compete. Often few are technically difficult to implement just a huge amount of work to get through and lots of nit-picky things like borders on controls rendering differently on different browsers and the like. You're not solving one of lifes mysteries but you still might burn a week figuring out why one does it different than the other and finding what magic combo makes all browsers happy at the same time.
Or rather, those features came out because the programmer involved didn't have to worry about a lot of stuff - type rendering, graphics, even a lot of the UI is already socked away nicely by the OS. Back in the old days, all that stuff had to be written from scratch, and if you're dealing with PCs, the hassles of configuring the various video modes and such.
Now it's abstracted away, and we can turn to adding real value to the programs rather than just boilerplate. A clone of notepad is trivial, which is the point - we can get the basics going with little work. Then we can add the hard stuff like syntax highlighting, regex and other things that go to the core of the editor, not fluff like how to display a menu and things we don't care about.