Tons of hobby operating systems exist, but hobbyists have built browsers too.
*Emacs has eww
*Webkit and Chrome were both born from KHTML, a FOSS project
*Servo started as a small research project
I admire hobbyists who build their own kernels, but the size of modern systems is intimidating. Even something like a Bluetooth stack is bigger than historic kernels, and tends to require multiple people.
I think it's just the cost of compatibility? Tech collaboration is hard.