Because the thing people refer to when they say “linux” is not actually an operating system. It is a family of operating systems built by different groups that are built mostly the same way from mostly the same components (which, themselves are built by separate groups).
1 line of code?
Amateur, I changed 1 byte of code in the Linux kernel!
It was random driver with something along the lines of “if (hardware_version > 3) fail()”.
One day we got a new shipment of hardware that wasn’t working for some reason until I upped that 3 to a 4.