Your forgetting (and I had to look this up) but the Enterprise-A was retired in 2293, while the Enterprise-B was launched later in 2293. Plus we know that the B was incomplete when launched and was missing a number of crew and parts. So the B was being constructed and was in testing flights (sure it wasn’t the prototype—-unless it was its subclass prototype) while it’s predecessor was out battling General Chang. Unless you are suggesting that the B was constructed in, what? 6 months from when it’s keel was laid to what we saw in Generations?
I wasn't suggesting it, because I didn't bring it up, you did

Here's what I think happened with the Enterprise-B: In STIII, when Admiral Morrow told Kirk that the Enterprise was going to be decommissioned, there were already plans to replace her with a new Excelsior class starship, which would have been the Enterprise-A. This was 2285, so there would have been eight years between the original Enterprise's decommission and the new Enterprise's commissioning in 2293. However, after the whale probe incident, Starfleet gave the Ent-A name to another Constitution class ship in the meantime, while construction started on the Excelsior class ship, which would now be called the Enterprise-B. Once the B was complete, Starfleet decommissioned the A.