For many (most) types of objects lifetimes can be a runtime property just fine. For e.g. a list, in rust/c/c++ you would have to do an explicit decision how long should it be "alive", meanwhile a managed language's assumption that when it's reachable that is its lifetime is completely correct and it has the benefit of fluidly adapting to future code changes, lessening maintenance costs.

Of course there are types where this is not true (file handlers, connections, etc), and managed languages usually don't have as good features to deal with these as CPP/Rust (raii).