Because we hit the carrying capacity of Earth, under the current socioeconomic paradigm.

Its too expensive now to have kids, broadly.

This will continue as food/water/energy prices explode due to oil becoming more expensive, regardless of Hormuz being opened or not.

More specifically, if oil prices spike hard enough and fast enough, we get an accelerating complex systems collapse, like the bronze age collapse, but of greater magnitude.

Shift or wiggle those lines around a bit, but yeah, something like that is what we're heading into.