Strong difference in companies coming for SI and Placements

Why is it so, that nearly no Day 0 SI companies come for Placements, and only some select Day 1 companies come.

Ofcourse not considering the PPOs.

Doesn't this practically make SI the only way to get into "top" tech/Quant firms, in particular. With PS 2 or Placements not matching the intensity.

( Talking wrt Pilani campus, idk if similar concepts exist at Goa and Hyd)