Forerunners have no need to fight ground battles the only enemy that forced them to do so was the Flood and even the Armigers and Promethean's where generally dropped from orbit into flood ships which they then cut their way into. Ship and production speed is the Forerunners main assets being able to produce single attack fighters and war sphinxes en mass in minutes they can swarm and destroy an enemy in a single surprise attack. Especially when said fighters and war sphinxes have enough fire power to vaporise cities and up turn continents.