I've always seen the reason for egg crate as being as much about keep any rocks placed on it stable, as about static weight distribution.
As highlighted in other responses cichlids dig and if they undermine the rocks the rocks will shift and you can end up with a rock slide and depending on the size and shape of the rock you can loose the front of a glass aquarium very easily (Iâ€™ve seen the aftermath of a 1,000 liter break and it wasnâ€™t pity). Egg crate will provide more grip for the rocks than a smooth glass bottom, which will help prevent rock movement should the fish move the substrate.
Yes, you can use silicon to stick the rocks, but Iâ€™ve not always found that to be highly reliable, especially as it ages.
If you can't get egg crate then you can use Koi filter plastic mesh
Just my $0.02