That was a weird era in Java history. They changed the compiler in the subsequent major version to perform that transformation automatically, but by then people had had 2 years to stare at perf graphs looking for bottlenecks.
It never was clear to me why they didn't do both of those in the same release. Backward compatibility wasn't the problem (they were already breaking that left and right).
Even better, I think Eclipse’s Java compiler introduced the optimization in a given version, but Maven hadn’t yet. So it wasn’t optimized in production, but was optimized on the developer’s machine. What a time to be alive.
Java devs saw that "abc" + "def" involved expensive String concatenation, so as a performance improvement they pro-actively, and effectively manually, changed to use explicit StringBuffer concatenations.
When the compiler switched to generate StringBuilder (unsynchronized) concatenations for "abc" + "def" nobody benefited, because they had already changed to use StringBuffer (synchronized).
Now they had to go an undo all of their hard, manual, optimization work.
It never was clear to me why they didn't do both of those in the same release. Backward compatibility wasn't the problem (they were already breaking that left and right).