It is caused by an undesirable defect of all living beings. Most of the structure and constituents of the living cells have their origin before large concentrations of free dioxygen have appeared on Earth, and they are easily damaged by it. The living beings that have adapted to live in the presence of dioxygen have acquired many adaptations to protect their sensitive parts from the action of the dioxygen, but these adaptations are not perfect and a little damage may still occur.
Unlike in certain bacteria (the methanogenic archaea), which transform a part of their food into methane to produce energy, this new method of methane generation does not have any advantage for the living beings where it occurs.
On the contrary, it is a byproduct of a process by which the ambient dioxygen destroys a small part of the living matter, which is a loss for the cells where it happens.
This methane is like the soot from an incomplete burning of some organic matter.
Not in the context of methane generation, but I'd imagine there's scarcely a discussion of climate change that doesn't include the impact of humans in general, and human population growth in particular.
I haven't seen it done so much, maybe not ever. I'm biased, of course, from reading studies that interest me.
Rule of thumb, human body footprint is negligible. The top 10% wealthiest humans account for 50% of GHG emissions, and top 1% account for 15% of GHG emissions. (See https://www.oxfam.org/en/press-releases/carbon-emissions-ric...)
https://www.bbc.com/future/article/20210805-the-search-for-t...
https://www.epa.gov/ghgemissions/overview-greenhouse-gases
I think I read somewhere that if we simply composted near 100% of all compostable organic waste that it would reduce methane production by around 8-10% (from memory). So basically a trivial solution can't even be enacted by humans.
The reason it's no longer the hot topic is because industrialised nations' populations do not grow - as living standards increase, birth rates approach parity or slight shrinkage.
So the good news is that to control population growth we should do the thing that we should be doing anyway: get the whole world up to a sustainable high living standard.