Why would we want to do something the compiler can do? There is no bragging rights to stupidly doing what a compiler can do. I can know exactly the scope of some malloc'd buffer, but so can a wrapped type like a vector. Why would I possibly want to waste time writing and testing/valgrinding to make sure every malloc is freed when the compiler can automatically call the destructor for me?
Because you have information that the compiler doesn't have. Like what's vital code and what's gilding, and how to prioritize based on business needs. You can free up non-critical code allocations early to avoid background gc impacting performance, or re-purpose already allocated memory in critical code to reduce overhead, or a large number of other techniques that are available because you know more than the compiler.
Why do anything that a machine can do? Because in many circumstances you can do it better. Use automation where useful, but when you have knowledge that the automation lacks, use it. Whether you're driving a car, cooking a meal, composing a song, or writing a program, use your special knowledge and skills. If you think a black box can do it better than you can, you're probably right. And ripe for being replaced.