How so? As long as your code is also being used, you can sue for copyright infringement on your work. I don't see why you need to call up anyone else.
I'm not a lawyer and can't explain why things are the way they are. I can only inform you that this is the way things have shaked out in the past.
A prime example is the SFC vs. VMWare lawsuit, where SFC represents a single contributor to Linux Kernel (Christoph Hellwig).
No, if you hold a valid sublicensable license (which includes any F/OSS license, as that's rather the point of Free Software/Open Source Software), and make a derivative work as authorized by that license, you hold the copyright on the derivative work and can enforce it without any participation by the upstream copyright holders.
You may run into difficulty if the offender can show by a preponderance of the evidence that their allegedly-infringing work is derived from the upstream rather than your derivative work, in which case the upstream would have to take action.
This is demonstrably untrue - all the Linux kernel GPL enforcement cases have been brought by individuals without involving any other kernel copyright holders.
While your assertion is true, it's not the only reason (or even the main reason) for CLAs in OSS.
IANAL, but this has actually happened in the past (the big case I'm aware of predates open source but not free software -- google for Diku MUD). Let's say that someone infringes on the license. So you take them to court to get them to follow the license. If you have a CLA you are the copyright holder so you can just do this. If you don't, to make your case you have to round up virtually everyone who has ever contributed to your project because they are copyright holders too. So, in effect, it doesn't matter if they infringe on the license or not because you can't hold them to it.