"Android is Linux" is a bit oversimplified.

What the is issue, still simply, the way I understand it:

• Linux kernel contains drivers for the specific hardware used in devices (processors, modem, memory, display, camera, etc.)
• Each Android smartphone has different hardware configuration
• Hardware manufacturers want to guard their secrets, so they sign contracts and NDAs with phone manufacturers
• Phone manufacturers create a unique, dead-end fork of a Linux kernel that contains drivers and is configured specifically for that model. (There are exceptions, but generally)

So yes, Android uses a Linux kernel, but in most cases, a very specific one.

Why not replace it? This requires:

• Access to the bootloader and ability to read/write to internal storage on low level, and manufacturers lock it down.
• Knowing the hardware and the drivers. As mentioned, manufacturers will provide drivers only to their contracts. So someone would need to write a driver.
• Once someone writes a driver, it can be added to mainline Linux, available to all.
• That is why "mainlining" a device is a big deal - that means that the kernel for that device can be built, and going forward, that device will be supported for all future kernel versions.