Why are there no spot rounds for IIT UG?

BITS, IIITB, IIITH and several other counselling are ongoing and will continue until July End/August, but JoSAA counselling for IITs closes by July 21st. What happens to all those seats in IITs that are left vacant by students joining other colleges? Isn’t it a waste of resources leaving all those seats vacant? Am I missing something? Can someone throw light?