Hi Raj,
I'm not an expert, but I doubt there is enough undeveloped soil around to absorb all that water. There are places that certainly can/could do it, but I dare to say that in most places the run off from the roofs is connected to the sewer system. There is so much concrete, asphalt/tarmac and buildings everywhere ...
I tried to look up some numbers, what seems to be fact for 2010 is that: 78.1 million people in Germany were connected to the public sewer system leading waste water to treatment plants/sewage plants. At the same time there were 81.8 million people living in Germany. Of course that doesn't mean that all of the 78.1 million people live in places that lead run off water into the sewer system, but again, I dare to say it would be the majority - and especially in and around all types of cities.
There was a time when people argued about wether to mix rain water with waste water in the sewer system or not. So some places still have two different sewer systems. One for rain water, one for waste water. But todays consensus is that most rain water, once it hit the streets, roofs ... is polluted and should be treated as well as/ like waste water. Originally the idea was - like you say - that rain water didn't need any treatment.
Personally I think using rain water for watering the backyard was a good choice, as was putting in these underground reservoirs. If you'd ask me today, I'd still do it different though. More accessible, more repairable more maintenance friendly.
Our situation is even a little more "painful" because the connections for both downspouts are lead back "into the house" and connected behind the the official hand over point. No one would do that in todays world, totally crazy to bring water from the outside back inside and leading it outside again. But that's how it was done back then.
But generally installing the reservoirs lead to less pressure on the sewer system which certainly saved us and others from flooded basements over time. But then again, when it's as bad as it was 2 months ago, all bets are off.
I guess, you and also others will be surprised when I document the repair of what was the real issue two months ago and where the water came from.
Kind regards,
Oliver