Regardless of your reasons for creating a bootable Windows 10 USB drive manually, here are the steps on how to do it.

Starting with Windows 7, Microsoft made it possible to install Windows without using an optical disc drive by loading the installer onto a flash drive.

1] Create a bootable recovery drive On a PC that’s already running Windows 10, plug in a USB flash drive and create a bootable drive using the Windows Recovery Media Creator.

That's it. Once the Windows 7 files have completed copying, insert your USB flash drive into a computer and boot it up to start the Windows 7 installation.