I agree that using SAM can be convenient for standalone lambda functions. However, in the context of a larger AWS configuration, where your integration points are already in defined in terraform, the cost of using SAM for these is breaking the invariant that "all our AWS resources are defined in _TOOL_ (in this case, _TOOL_ is terraform).
The verbosity is frustrating, but largely ignorable. Having to go back to string references of AWS references defined in different sources of truth is a maintenance nightmare, IMO