Any number of reasons. My guess at the time was that the food scientists who developed their 'flavor' figured that a certain percentage of the minerals would deposit on the inside of the bottle, and calculated how long it would take for the bottle to arrive at their desired concentration.
It could also be that they expected very slow chemical reactions to occur over time. Or that the minerals are added to the bottle after the water, just before the cap is installed, and they expect it to take time to distribute evenly.
It's hard to think of reverse-osmosis purified water with mineral sludge added as being "fresh" at any point in it's shelf life, but on the other hand i guess nearly all the water on this planet is technically as old as time itself, isn't it?
All I know for sure is that it's tough to find a bottled water that tastes as good to me as what comes out of the tap at my parents house, which is reputedly pumped from a spring up the mountain side, the mountain being chiefly composed of limestone. Some significant portion of that preference is probably because i grew up drinking it. A lot of this stuff is subjective.
Lindon UT. It's not the worst I've had, the water out in Delta tasted like someone had steeped cardboard in it. And i hear stories about restaurants in some town around here that had to give up plastic pitchers for glass after the plastic ones were stained green by something in the water.
The drinking water in Utah is generally excellent, if you're close enough to the mountains. Lindon is on a low spot at the north end of a bench near the lake. I'm not sure exactly where their water comes from, but it can't be anywhere good.
Much of Provo is literally drinking glacial spring water. Though in fairness, in recent decades, there isn't much of a glacier.
Out here in Orem, on the high part of the same bench, with a river between us and the mountains, the water typically has pretty questionable origins (the EPA term is 'surface water'), but it's never been as fragrant as what comes out of the tap in places like Antioch CA.