There's really no way to separate the success of the Arkham games, the Mordor games, Lego games, etc. from the IPs themselves. As consumers we might talk about the quality of the products, but from a business perspective there is very little you could do to prove that Rocksteady could produce a game based on a different IP that would be just as successful in the market.
Rocksteady is a well functioning machine that produces Batman games. You just can't prove that Rocksteady's Attack on Laser City is going to sell as well as Batman. If you take away Batman, the risk goes up and the value goes down. Same with the other studios. Stripping Rocksteady of Batman significantly strips the studio of value. If AT&T did this, it would make their own asset worth less.
Likewise, selling rights to a Batman game is a tougher sell than selling both the rights to make a Batman game and a studio that is extremely good at making Batman games. Even a company with lots of resources would be taking a risk that they will be able to produce an attractive product with a given IP (ask EA about Star Wars). Selling the IP and proven studios together would command a much higher price than either alone.
AT&T is trying to extract value from Warner Games AND its own IP, so it makes sense that the deal would include the right to make games (for some duration of time) to get the maximum value out of both assets. AT&T owns the rights to nearly all of the IPs in question (I think so far only LOTR is a bit ambiguous, but AT&T does own New Line Cinema...).
One final point that has already been raised is that studios that have no right to make the games they've been making are definitely not worth $2-4 billion dollars. That's just not the guaranteed value they would bring.