You know another project with much of its source files in the top-level directory? https://github.com/git/git
I don't think putting stuff in an extra src/ dir was very common in 1999. Pretty sure that became popular with websites like github. And once you have started without one, there is fairly little incentive for this project to change.
It was pretty common with Sourceforge era in the early 2000s. I agree (I think) that there isn't much reason to have one.
I like it that way actually, as long as the program isn't very large. Now I don't have to hunt down a directory tree to find the interesting bits.