I guess one significant reason is that it is not deemed necessary. Two important statistics in population growth are
- Total fertility rate, roughly the number of children a woman is expected to have if she was to experience the exact current age-specific fertility rates (ASFRs) through her lifetime and did not die before the end of her reproductive life.
- Net reproduction rate R0, roughly the number of daughters a woman is expected to have if she was to experience the exact current age-specific fertility rates (ASFRs) and mortality rates.
An R0 of exactly 1 means that the population will neither grow nor shrink. For the TFR, that same number is not exactly 2 because a) sexes are not distributed exactly 50:50, and b) some girls die before reaching childbearing age. For modern civilization with adequate healthcare and low child mortality, the number is believed to be around 2.1. (Just as an example: in the UK, it is 2.075, whereas in developing nations, it can be as high as 3.3.)
Because of these uncertainties, R0 is the more reliable indicator, but the UN stopped reporting it.
Note: An R0 < 1 does not necessarily mean that population will shrink immediately in the short term, for example if at the same time the life expectancy increases. However, in the long term, R0 < 1 means that the population will shrink, unless it is replenished from the outside, through immigration.
It turns out that many so-called "Western" nations are actually below those numbers. In other words, their population is already shrinking without the need for any laws or constitutional amendments.
For example, the average TFR for the EU-27 was 1.55 in 2018, the highest was France with 1.88. While TFR has been slowly rising over the last few years, we are still a very long way from even population stability, let alone population growth. US was 1.64 in 2020, Canada 1.46. South Korea is only 0.64, Japan 1.34, China 1.30. The entirety of East Asia is below 2, with North Korea the highest at 1.95 (but with higher child mortality rates compared to e.g. the EU). TFR for India is dropping rapidly from 3.2 in 2000 to 2.3 in 2016, 2.2 in 2018, and estimated 2.1 in 2019. Russia is at 1.5.
OTOH, if you look at the countries that are on the top of the list, then you have the entirety of Africa (with Niger on top with an average 6.8 children per woman), Yemen, Iraq, Palestine, Pakistan, Egypt, Afghanistan, etc.
[Source for all TFR numbers: either the Wikipedia article on TFR or this list on Wikipedia]
But TFR is not everything: for example, Mongolia has a TFR of almost 2.9, but does that mean that they are an important driver of population growth? Mongolia is the most sparsely populated country in the world with only 2 people (3.3 million total, slightly more than Chicago, slightly less than LA, less than half of New York City) per square kilometer (1.5 million). They could literally increase their population by a factor of 10 and would still be one third less densely populated than the US.
So, for lots of countries, especially the developed nations and so-called "Western civilization", it is simply irrelevant because a) it doesn't impact them (they tend to live in the fertile regions with lots of water, and worst case they can just buy their food from overseas or research themselves out of the problem with e.g. GMOs, synthetic food, hydroponics, vertical farming, etc.) and b) they can't really do much about it anyway, because their populations are already shrinking, so there is nothing to reduce.
OTOH, many of the countries with the highest birth rates are developing nations with barely functioning governments, very low education, and widespread poverty where families need to have many children because a) many of them die and b) they need their workforce to support them when the parents and grandparents become too frail to work.
If we want to look at it from a very simplistic, grossly exaggerated view, we could say that the countries with a functioning constitution don't have growing populations and the countries with growing population don't have a functioning constitution.