Your M1 Garand, M1 Carbine, and SKS most certainly do have forward assist, as they have charging handles that are directly connected to the bolt. That is where the idea came from for the forward assist, as Stoner's original design did not include the forward assist.

On the AR-15, the charging handle moves independently from the bolt. Stoner's original intent was to use your thumb in the scallop of the bolt to assist the bolt forward if needed. If it took more pressure than that, then pull back on the charging handle and eject the round. The Army wanted something that could be pushed forward to force the bolt closed, like the action on the M1 Garand, M-14, etc, so the forward assist was added.