Contemporary out-of-order CPUs are incredibly over-provisioned; microarchitects will justify a new feature by another 0.1% gain on some benchmark. The end result is a CPU that's pretty decent at handling the sort of code bloat that comes from a binary translator.
There's also a big tradeoff in adding more optimizations to a binary translator. You would like to be able to precisely handle exceptions (especially ones caused by invalid memory accesses) while presenting a userspace exception handler with an architecturally valid state for the source program. There are some optimizations that would be easy to do in principle but are painful for maintaining this mapping between source program states and translated program states. The complexity burden combined with the difficulty of debugging that added complexity (or exhaustively verifying it up-front) shapes many of your decisions when writing a production binary translator. You should always have more Cool (tm) ideas than you actually use in practice.