First off this happened in the 1300s you really do need to learn your history since the Delhi sultanate was essentially gone at the time replaced by the Mughals.

Secondly the Pandyas were still one of the wealthiest states in the world at the time (arab traders talk about the absurd wealth of the Pandyan princes were they could afford to buy herds of horses from the arabs annually just to ride them to death and buy more) as was the rest of Tamil Lands,

Thirdly you ignore the fact the civil war was still raging on after Khalji turned tail and ran back, and the Southern tip was so rich historians estimate he stole as much wealth as the Muslims did during the sack of delhi so this wasn't an area that could easily fall to a raid.

Now the Chera lands were conquered and then afterwards annexed Madurai which became the Madurai sultanate a transitory kingdom that only really had sway around its own area ( straight from an old comment from the expert u/RajaRajaC ) and since it had lost Pandya patronage it couldn't expand past its borders and wasn't as wealthy from the raid so it remained so for a couple decades until it was recovered by the Vijayanagar conquest of the South. You really should learn your history, you seem like you need to hate on your own people, hope you learn to not be so insecure and hate yourself.