Why Did God Create All Things in Six Days and Not Simultaneously?

I have been working on a presentation I am to make in February. My work constrains me to engage Calvin quite fully. Even though I've read the whole of Calvin's Institutes and portions numerous times, the following had not stood out to me as it has today. Among the various things to notice, one stands out. Calvin, before famous Bishop James Ussher, already believed that creation was about six thousand years old. This is so despite the fact that many appeal to Calvin to provide support to their theory that the six days of the creation account in Genesis are actually extended periods of time. Calvin believed that the six days were just that, six days, and that the universe is quite young. In terms of categories thrown around these days, John Calvin was a "young earth creationist."

John Calvin, Institutes of the Christian Religion, 1.14.1.

Isaiah rightly charges the worshipers of false gods with obtuseness,because they have not learned from the foundations of the earth and the circle of the heavens who is the true God [Isa. 40:21; cf. v. 22; see Comm.]. Despite this, such is the slowness and dullness of our wit that, to prevent believers from deserting to the fabrications of the heathen, we must depict the true God more distinctly than they do. Since the notion of God as the mind of the universe (in the philosophers’ eyes, a most acceptable description) is ephemeral, it is important for us to know him more intimately, lest we always waver in doubt. Therefore it was his will that the history of Creation be made manifest, in order that the faith of the church, resting upon this, might seek no other God but him who was put forth by Moses as the Maker and Founder of the universe. Therein time was first marked so that by a continuing succession of years believers might arrive at the primal source of the human race and of all things. This knowledge is especially useful not only to resist the monstrous fables that formerly were in vogue in Egypt and in other regions of the earth, but also that, once the beginning of the universe is known, God’s eternity may shine forth more clearly, and we may be more rapt in wonder at it. And indeed, that impious scoff ought not to move us: that it is a wonder how it did not enter God’s mind sooner to found heaven and earth, but that he idly permitted an immeasurable time to pass away, since he could have made it very many millenniums earlier, albeit

the duration of the world, now declining to its ultimate end, has not yet attained six thousand years. For it is neither lawful nor expedient for us to inquire why God delayed so long, because if the human mind strives to penetrate thus far, it will fail a hundred times on the way. And it would not even be useful for us to know what God himself, to test our moderation of faith, on purpose willed to be hidden. When a certain shameless fellow mockingly asked a pious old man what God had done before the creation of the world, the latter aptly countered that he had been building hell for the curious.

Later within the same section Calvin addresses anyone who would "raise questions concerning immeaursable stretches of time" as with "space." He states,

Now if anyone should expostulate with God that the void exceeds the heavens a hundredfold, would not this impudence be detestable to all the godly? Into such madness leap those who carp at God's idleness because he did not in accord with their judgment establish the universe innumerable ages before. To gratify their curiosity, they strive to go forth outside the world. As if in the vast circle of heaven and earth enough things do not present themselves to engross all our senses with their incomprehensible brightness! As if within six thousand years God has not shown evidences enough on which to exercise our minds in earnest meditation! Therefore let us willingly remain enclosed within these bounds to which God has willed to confine us, and as it were, to pen up our minds that they may not, through their very freedom to wander, go astray.