It can be: What definition to jump to if there are multiple (e.g. multiple Translation Units)? What if the function is overloaded and none of the types match?
With grep it's easy: Always shows everything that matches.
Sure, there might be multiple definitions to jump to.
With grep you get lots of false positives, and for some languages you need a lot of extra rules to know what to grep for. (Eg in Python you might read `+` in the caller, but you actually need to grep for __add__ to find the definition.)
With grep it's easy: Always shows everything that matches.