Because, without atgm how will you target those tanks ?
Very true.
Now just go back to the main reason for building LCH in first place.
It was built after Kargil war to take the war at those heights where conventional gunships or helis can't operate. For that the 20mm gun and 70mm rockets are enough. Same is true for any light tank deployed at that height. For them the ATGMs are kind of over kill.
But yeah, we do need them for sure. But its not like that it can't operate or would be ineffective without those ATGMs.
Now if we look into the delay of its induction, we would find that the weapon package is the main reason besides everything else. Although the gun and rockets were there from day one of inception. But the turf war in between IA and IAF caused the whole thing to go back into the chill plate. IAF simply didn't want IA to operate them just like Apache.
MoD played along the line and then the demand for AAM on board came into play and all the drama of Mistral integration happened. Now IAF is major stakeholder in the program. If you follow the news, IA is not insisting on ATGM integration in a stubborn manner. But they would take it if it comes along which DRDO is working on. Its the media experts who are towing the line that without an ATGM, LCH is toothless.