Is that because folks are pooping in the rivers? You’d think folks could just catch rainwater. Sounds like an infrastructure issue...

No, no large-scale pooping in rivers. Far more complicated than a single technological fix can handle.

Areas which get 2000+ mm of rainfall a year aren't always the ones with the most intensive agriculture. Water shortages are partly because of (a) the huge variation in the amount of rainfall from year-to-year in any area in India, and this variation has grown with climate change/warming (b) rivers down in the south of the country are not perennial but largely rain-fed during the monsoon. So "build more dams/infrastructure" doesn't make sense if there's not much rain to feed the rivers, (c) certain crops grown (sugarcane,paddy) just require tons of water, and are more likely to face issues due to variable rainfall compared to something like millet cultivation (d) rain is required not just for flow into rivers but for recharging groundwater, which is then used for agriculture. So, no rain = no groundwater and not just no rain = less flow in rivers.

Which highlights another problem with systems like these: Humans generally settle near sources of water already. The problems with water in poor countries is generally not one of water availability, but of water treatment.