Why do i need to sign in to use Windows?

This might help if this method still works: 

https://www.howtogeek.com/442792/how-to-create-a-local-account-while-setting-up-windows-10/

To create a local user account anyway, you’ll want to disconnect from the internet at this point in this installer. If you’re connected via a wired connection, unplug the Ethernet cable.

This is a question that is usually asked by people who pay no attention to what they are signing up for when they purchase something. Or in how their computer operates.

You buy a computer of one make or another. HP, Dell, Lenovo...whatever. You own that hardware.

You buy a license to use the Microsoft Operating System that is installed on it. Whether that be Windows 10 or Windows 8.1 or whatever. It is the same deal with Apple to SOME extent but you don't notice it so much because they manufacture the hardware AND the software. Which Microsoft usually doesn't.

Your LICENSE agreement consists of the legally binding obligations you enter into with the Microsoft Corporation. There is a copy of this on every computer Microsoft allows their operating systems to be installed on.

simply type license.rtf in your search window and press enter and you will see your full license which includes but is not limited to this:

Now you CAN sign in with a local account to "use" your computer unless you wish to use your Microsoft email account, Office Apps in general, the Microsoft Store, etc, You would still have to log in with your MICROSOFT account to use those.

Offline installation should still work without MS account...

But just recently some more weird observations were made... You might want to read: https://borncity.com/win/2020/02/25/forced-microsoft-account-also-in-windows-10-pro/