Once enough people have access to the software, it could be eventually leak. Then everyone's device will essentially be backdoored.

Right now we assume no one has developed the capability to exploit this weakness. By forcing Apple to develop this capability, the security of everyone's device is compromised. This assumes that Apple is uniquely positioned to develop the exploit, or at least they are the only ones who can do it quickly/cheaply enough to make it worth it.

At the very least, forcing apple to do it publicly demonstrates the feasibility of the exploit to third parties.