Answer:
Their life ideals were dedicated to fighting and defense, so they preferred to honorably defend the city with army and manpower rather than with the walls.
Step-by-step explanation:
Sparta had a good geographical position – they were position in the Taygetos mountain rage that provided them with the natural protection which would slow down the enemy. Ready for the defense as they were, Spartans were on the watch way behind the edges of their city. With the mountains and guards ready, the enemy would hardly reach the borders of the city before being spotted and killed.
However, the biggest reason was the mentality and ethos of Sparta. Spartans valued the belligerence and were very talented fighters. They would train their armies from a very young age and be always ready for the fight. They preferred to have the manpower for defense instead of the walls. It was their point of pride and honor to fight and protect themselves and their city borders.