By design, starting I think back to Windows 7 \ 8. Windows Defender is a part of the operating system. It's there without you downloading it.
For Windows 10 it's an improved package to be used totally as onboard antivirus (Some like it, some don't)

Now, many decide to download and install other antivirus to use and it's there choice and manufacturers even supply trial antivirus when selling new computers and this is OK because, Windows Defender will disable to allow these other applications to run.
If you want to experiment with other antivirus thats fine or if you want to try and use Windows Defender thats fine too.