These differences aren't inherent to transpilers vs compilers, they're mostly the result of the fact that the vast majority of transpilers are less mature than

These differences aren't inherent to transpilers vs compilers, they're mostly the result of the fact that the vast majority of transpilers are less mature than the battle-tested compilers that you're thinking of. The average hobby compiler—regardless of target—doesn't optimize code and works directly on the AST because that's simple to get started with. Most hobby compilers also target some other language rather than LLVM or machine code because that's simple to get started with, so the result is that most transpilers are hobby projects that don't optimize. But there's no reason why a transpiler shouldn't include optimization steps that adapt the output to use code paths that are known to be fast, and a production-grade transpiler typically will include these steps.