Because the word kernel means "core". Simply put, the Linux kernel is the core program of Linux. Strictly speaking, "Linux" = "Linux kernel".
Since the Linux kernel is the core program of the OS, it gives instructions to run the hardware.
A Linux distribution includes various programs such as the Linux kernel and the user interface (a program that mediates between the user and the kernel), and refers to a program that is ready for use by the user as an OS. Specific examples of Linux distributions include Cent OS and Ubuntu, which have recently been announced to be discontinued.
Recently, these "Linux distributions" are increasingly called Linux.
I would appreciate it if you could point out any mistakes.