Essentially, unlike the Linux on a PC or a server, every single Android phone has a very different Linux kernel built specifically for that particular phone, which is the reason that people need to build a custom rom (basically a device specific really old Linux kernel) for every phone instead of just installing something like a generic Ubuntu system.
Base Linux uses a lot of parts made by GNU, which is licensed under a strong copyleft clause, and Google doesn’t like that because they don’t like to share their source code, so, Google made an active effort to replace the GNU part of the base Linux kernel with something else to make Android to prevent that from happening.