Why is it not possible to open RAWs or at least DNGs with Nik Collection, why do I need to create a Tiff first, if I’m using Adobe Lightroom or DxO PhotoLab?

Nik Collection was never designed to be compatible with RAW files. It was originally designed as a plugin, therefore it is compatible with JPEG or TIFF images using RGB color space. We recommend using DxO PhotoLab as a host software for images from supported camera/lenses and convert to TIFF, or using DxO PureRAW to convert your RAW image from a supported camera/lenses to JPEG.