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It's a matter of repetition. The top guys get the most, and what's left gets filtered down.
Eventually, there aren't any available.
Despite calls for QB Jerrod Johnson's addition to the Practice Squad, NFL teams don't typically keep quarterbacks there, as it typically means they have four quarterbacks. When you have an alpha-starter like Roethlisberger, the snaps to take in practice are already scarce for the back-ups. Going from three to four in that regard would serve only to take snaps away from the top three quarterbacks.
A wiser course of action would simply to be to tell him to stay in shape and stay by the phone.
This is based on the bigger picture, but recent Steelers history has shown the necessity of being prepared for short-term quarterback quandaries.
Not that roster cuts should be made solely based on emergency situations, but in the unlikely and horrible event both Ben Roethlisberger and Byron Leftwich went down with injury, it's a near certainty Batch would get the start over Johnson anyway. Batch is 5-2 as a starter, and this isn't a team that throws starting quarterbacks into the fire without reason.
Not that it hasn't produced results in the past, though.
Take Roethlisberger as an example. His rookie year began with him being third on the depth chart. Batch, the back-up to Tommy Maddox, went down in training camp, elevating Roethlisberger to the second spot. When Maddox got hurt, Roethlisberger went in, and the face of the franchise changed.
Before that, Maddox replaced Kordell Stewart in the second half of a Week 3 game against Cleveland. Maddox led a comeback win that prevented the Steelers from starting in a disastrous 0-3 hole to start the year.
Leftwich replaced an ailing Roethlisberger in the second half of a 2008 game at Washington, taking a 10-6 nailbiter to a 23-6 blowout win on the road in primetime.
Dennis Dixon and Batch both contributed to wins in 2010 while Roethlisberger served his suspension. Dixon did enough to beat the Falcons and both he and Batch contributed to a win over Tennessee in Week 2. Batch started and got the win against Tampa Bay in Week 3, while suffering the loss to Baltimore in Week 4 before Roethlisberger's return in Week 6 (their bye was in Week 5).
The decision to go with just two quarterbacks is an understandable risk, and doing so could lead Johnson to be signed to the Practice Squad. It just seems more likely they'd tell Batch to be ready by the phone, because in the event they brought back a quarterback, they'd want the experience more than they'd want the potential. Johnson being on the Practice Squad would imply they want him to grow into a future back-up to Roethlisberger. He'd need snaps that just wouldn't be available - or at the very least, he'd need far more snaps than Batch would throughout the season.