What's a good reason for a nation that's mostly desert to not make a massive effort to collect all the rain water that falls in the small forested regions of the nation?

So I have this nation (Nefaria) of whom its southern parts are a very dense forest that gets a lot of rain. It's flat and right next to where the nation's longest river begins. But north from that, all that remains are a few hills and a massive desert that covers more than half of the nation. The nation is a pretty poor and small country where most people live in small settlements along the river. In the desert, there are only a few oases where people live. I'm trying to create a situation where the nation really lacks clean water, which I think is a much bigger problem than a lack of food (because the river is plentiful in fish), but I seriously can't come up with any good reason as to why the nation won't go all out in trying to collect all the rain water that falls in the forest to get a supply of fresh water to then distribute along the river (and maybe the oases too). As far as I can see, the only real problem with this is the cost, but that would be a huge investment that would pay for itself in no time, so I'm not really seeing why it wouldn't be done. I want the nation to really lack water, mainly because I want them to see it as the real source of life (instead of fertile land and a good climate which would give them plenty of food), and obviously they won't see water as so valuable if they have plenty of it. So help me please!<br><br>Edit: thanks to everyone who commented! I really was really close to breaking down and crying on seeing I got a reply. But when I actually read it all, I got a lot of good points and ideas to make my world more interesting!