Because God exists outside of time, and He can do whatever He wants, whenever He wants...or not at all. He does not need our permission, nor is He required to let us in on all the details. I believe and trust that God reveals the correct interpretation of Scripture to each believer that asks for His guidance. I trust Him far more than I trust humans.
So you and I both have access to the SAME DATA about creation, data that is EXTERNAL to us, that can be examined and discussed. And, to a Christian, the issue is seldom what God can do, but what He has done, or has promised that He will do.
And so in this case. We have the data. God states He made the universe in six days, days identical with the six-day work week He enjoined upon Israel (Exodus 20:11). So, regardless of how we feel about it, you and I are both left with identical objective data, to wit: God states He created all in a 6/24 period. As Christians, we pledge to believe Him.
So I still wonder how do you fit the hundreds of millions of years of supposed processes or lurches guessed at by non-eyewitnesses within that time-frame?
Dan