Well the point of gc is to allow the algorithm to focus on the problem domain. and not worry about mechanical implementation details of a computer. Everything has a trade off. An obvious win still must pay a cost elsewhere. In the case of GC it's simply not a good trade off for many resources. Sometimes it cannot be used even if you were willing to accept a trade.

Managing resources by hand is managing the mechanical implementation details of a computer. So in order to have garbage collection that manages more than just a sliver of memory so small that it counts as a rounding error by the same you get to the size of real world projects, you need a custom operating system and possibly even custom hardware too.

Your statement is false for many real world projects. (For some, you are correct. Those projects should not be written in a garbage-collected language, or should be written to run on a custom OS and custom hardware.)

When GC'd objects represent resources like open connections and files, use-after-free safely and reliably raises an exception to handle. Without GC, use-after-free on pretty much any object is so catastrophic that even correct code starts doing unpredictable things.