Because there's no thing as "separate but equal". It hasn't worked, it never works. If something is legally identical, then it's the same thing - as your post notes!
In most countries, "marriage" is the legally protected definition of a civil union in the eyes of the government. As soon as there's meant to be a "same but different" construct, prejudice creeps into the enforcement because legally things being "different" is interpreted as allowing difference.
The idea of "civil unions" being different to "marriage" has been used to do things like deny medical authority to one partner because they believe they might successfully litigate it. The only reason to try and draw a distinction has been to try and insert a seam where prejudice can be re-injected[1].
In most countries, "marriage" is the legally protected definition of a civil union in the eyes of the government. As soon as there's meant to be a "same but different" construct, prejudice creeps into the enforcement because legally things being "different" is interpreted as allowing difference.
The idea of "civil unions" being different to "marriage" has been used to do things like deny medical authority to one partner because they believe they might successfully litigate it. The only reason to try and draw a distinction has been to try and insert a seam where prejudice can be re-injected[1].
[1] https://www.aph.gov.au/DocumentStore.ashx?id=4726e827-5d5a-4...