Isn’t Unity’s code available? Unreal’s is, at least. It’s not quite the same as having official support and you don’t have the same rights as in open source, but you can at least change the code and fix it yourself if you really need to (with the difficulty of understanding a complex codebase, but that’s the case in open source engines too)
The other commenter answered it (that unity’s source costs money to get access to), but my comment wasn’t about whether it’s proprietary or not, but rather if the source is available/accessible. Unreal’s source code is on GitHub, but it’s still proprietary. You cannot redistribute it or anything like that.
Unity is a lot more choosy about who gets their code than Unreal. To get the source you need to be in at least the $1800/seat/year tier and pay an additional cost that isn't specified.
