If there’s a New Covenant and an Old Covenant that has passed away, why do Christian Bibles contain the Old Testament?

We’ve been working our way through Matthew 5, the first part of Jesus’s Sermon on the Mount. We have looked at a very pivotal verse, I believe: Matthew 5:17, where Jesus told those who had gathered that day to hear Him, “Don’t think I came to abolish the law and the prophets. I didn’t come to abolish them but to fulfill them.” Then He says in the very next verse, “Truly I say to you: Until heaven and earth pass away, not the smallest letter or stroke shall pass from the law until all is accomplished.”