Finder favorites bar icons not showing
This's actually by design since macOS has stopped showing custom icons in recent versions of the OS.

My Google Drive icon is also a generic icon, others too:



If you want to display the Google Drive icon, it wouldn't be possible without third-party software.

Extra: Dropbox uses a "hack" to show its icon in the sidebar as seen in this question, How come Dropbox gets an icon in Finder sidebar?.