There are two simple reasons for this. First, there is the cost of price. It is much cheaper to create a park with green grass and a few things that people can enjoy than to create a water treatment center or a sewage center. So, there is the issue of price.
Second and more importantly, no one wants a sewage and sanitation center in their town. There will be an uproar from the community and protests. This public pressure is a very real concern and hindrance from having a sewage and sanitation center. For example, last year the pipe of a sewage center in New York City broke and all this sewage went into the Hudson River. Who wants this? However, would would say no to a park?