According to Hebrews 8, the old covenant had something wrong with it because of humans breaking the covenant. How do you respond when asked, "Why did the all-knowing God create a covenant that would become obsolete when He knew the people would break it?" Would this be an instance where we don't know why God did what He did, but we simply know what God did?
Why did he create Israel and the old covenant? Probably for the same reason that he created the human race in the first place. It all fits into his plan for a relationship with us, his creation.
The Lord does not rush things. His normal working is through processes, not instantaneous fiat.
But maybe you are right, and humility dictates that we be content with no comprehensive answer. Still, if one studies and falls in love with the Old Testament, I think his opinion will change.
For an introduction to the world of the Old Testament, just click.