The main reason is Windows 7 using EHCI structure which are incompatible with XHCI structure on Skylake, Kabylake, Braswell and AMD Ryzen AM4 platforms. Therefore, USB 3.0 is out of work when you attempt to install Windows 7 on platform.
In order to address this issue, we should add XHCI driver into Windows 7 installation disc automatically integrating USB drivers from Skylake, Kabylake, Braswell and AMD Ryzen AM4, then, produce USB files for installing workable Windows 7.
ECS Windows 7 Image Tool can help you to modify Windows 7 for these platforms.
Please download Windows 7 Image Tool follow the steps on the below file to get it done.
Download Link
http://download.ecs.com.cn/dlfileecs/faq_file/ECS/eSW7_v1.0.1.1.zip
How to use
http://download.ecs.com.cn/dlfileecs/faq_file/eSW7_HowToUse_Eng.pdf