This is a good question, but it’s predicated on a profound misunderstanding. Why didn't Hitler just cut off the water to the camps so there would be no survivors? Alternatively, why didn’t Hitler (or, more accurately, Himmler) start sending everybody within the camps to gas chambers? If the intention were that nobody would survive, why are there so many survivors?
Between 1933 and 1945, the number of camps that the SS administered in Europe numbers in the vicinity of 40,000. A great many of those camps only operated for a very short period of time - sometimes as little as a few weeks. Some of those camps (like Dachau, for example) operated for a very long period of time, and underwent a number of transformations.
Camps were places of punishment and “re-education”, and ultimately places where SS guards could practise their brutality upon a population that in time they came to despise with an intensity that cannot be understood outside of the camp environment. These were places of mass murder, but that mass murder had its limits.
Groups that experienced complete (or near complete) annihilation within the camps included Soviet POWs, who largely were exterminated through the denial of food and water. All told, somewhere between 300,000–500,000 Soviet POWs were murdered each month, from October-December, 1941.
From 1942, Oswald Pohl took over from Theodore Eicke as chief of the Concentration Camps Inspectorate (CCI in English sources; in German, the Inspektion der Konzentrationslager, or IKL). Pohl was head of the SS Main Economic and Administrative Office (SS-WVHA), and under his control all concentration camps became glorified labour camps, where prisoners were expected to work towards the German war effort and stimulate the German economy.
Prisoners were somewhat expendable, and notwithstanding cadres of privileged prisoners, were also easily replaceable. Conditions within camps were horrific and punishments draconian, but the general bulk of the prisoner population, as a whole, was expected to soldier on. The two exceptions to this rule, despite the fact that one of them fell under the jurisdiction of Oswald Pohl, were Majdanek and Auschwitz.
Majdanek and Auschwitz, in addition to belonging to the concentration camp system, also played host to another type of camp, and one that (as a general rule[math]^1[/math]) fell under the jurisdiction of Odilo Globocnik. In addition to Majdanek, the camps that were overseen by Globocnik were the so-called Operation Reinhard death camps: Treblinka, Sobibor and Belzec.
Unlike the concentration camps, these were places where the arrivals were murdered, and where (with the exception only of a small group of workers who were granted a temporary reprieve) there was to be no surviving prisoner population. So effective were these camps that the number of survivors is negligible, while the number of people who were sent to them is astronomically large.
The starkest example is that of Belzec. Of the 500–600,000 people who were sent to Belzec, the number of “survivors” may be as low as 2. One of those two people, Chaim Hirszman, was murdered in Poland in 1946.
Both Treblinka and Sobibor had prisoner uprisings in 1943, at which a number of people escaped. Some of those escapees survived the rest of the war, meaning that the number of survivors (about 87 for the former, and around 50 for the latter) is a little larger.
This phenomenon, by which certain groups of people (Jews and Roma, specifically, but also - outside of the camp system - people with intellectual disabilities and mental illness) were slated for complete annihilation is something that historians refer to as the Holocaust. It is different, both in its motivation and its unfolding, from the other abuses perpetrated by the Nazis and their collaborators - including those with a shockingly high death count, such as the murders of Soviet POWs and Poles.
Where it comes to the latter, as well as all those who were sent to concentration camps (gay men, political prisoners, sex offenders, etc), the idea that somebody might have just “turned off the water supply” overlooks the fact that murdering every single person within the camps was contrary to what the camp administration was seeking to do. Murder was a commonplace, but mass murder on that scale (excepting the Soviet POWs in 1941) was not.
Where it comes to the former, turning off the water supply to places like Sobibor and Belzec would also be counter-productive. It takes about a week-or-so to murder somebody through sustained water deprivation. The Nazis had a far more efficient system.
[math]^1[/math] Globocnik oversaw death camps a general rule, but not exclusively. Chelmno was a death camp of sorts, but did not fall under Globocnik’s jurisdiction. Neither did Auschwitz-Birkenau, which despite its being undisputedly a death camp remained within the concentration camp network as overseen by Pohl. It was different to the other camps that Pohl administered, in that Auschwitz had a large population of Jews: a group of people who, by and large, were not sent to concentration camps.