Once God has made the covenant, He no longer speaks in the future tense. He does not say, "I will give." He says, "I have given." The covenant has settled it finally and forever.
The reason that God entered into covenant with Abram was to express His mercy to mankind. The definition of mercy is to withhold from one that which he deserves. This is why God placed the Mercy Seat in the Tabernacle.
The procedure by which the Lord entered into the covenant with Abram corresponds exactly to that described in Jeremiah 34:18-20. Abram had asked for a sign from God to help him with his unbelief. The sacrifice was used as the sign. Abram had to take the sacrificial animals, kill them, and divide them into two pieces. By that strange act, the Lord entered into a covenant commitment with Abram.