But they can only be globally unique if each instance has its own range of unique ID’s, otherwise they’ll have to check with the other instances to make sure the GUID they want to use hasn’t already been used. With new instances spinning up all the time you can’t really manage this.
I agree that the @instance provides a little more info, and it fits nicely in line with how user profile and community URLs are handled.
There was also a github issue report about putting the title in the URL, like reddit does, but I think this goes too far - lemmy has the ability to change the title and putting the title in the URL would just confuse things or lead to exploits (eg you put naughty words in the title then change it afterwards, but the URL still has the original title).