Just to expand on that, electromagnetic waves(which include light and radio waves) travel significantly slower on fiber than in air. Hence the high frequency trading people have been setting up microwave radio links to bypass the fiber links everyone else is using and get a few microseconds advantage on them.
To add to that -- microwave towers have their locations chosen to provide a straighter (ie shorter) path and subsequently a lower total distance to travel than fiber. Even if right of ways and logistical issues of the path of buried or hung cable isn't a problem, you still have to deal with the general topography of the earth. When all factors are considered, fiber links are far from straight lines, thus the data has to travel a much longer path while simultaneously traveling that path slower. RF saves milliseconds in a race where microseconds count.