It’s a bit of the shopping cart paradigm: no one is stopping you, but it’s a bit of a dick move since GOG literally allows you to keep perpetual copies for yourself forever.

Also, because of that exact scenario, many high profile steam game publishers are wary of being sold on GOG - and I suspect this exact instinct in the community is exactly why they are going to avoid the platform as long as possible. Even if it’s a repack, it’s a lot more work to find that release than to just use a GOG installer, and that ease of piracy scares them.