Because the goal of the Final Solution was the eventual murder of all European Jews, not their immediate murder, and no one has claimed otherwise.

The immediate murder mostly applied to the Soviet Jews in 1941-1942 (and then with lots and lots of exceptions) because they were thought of by the Nazis as the especially dangerous carriers of "Jewish Bolshevism", and was sometimes applied later (e. g. in the Aktion Reinhardt camps, albeit workers were sometimes selected there too).

The exploitation of the Jews able to work for slave labor was an important part of the Final Solution from the start and went hand in hand with the direct extermination. In the Wannsee protocol we read:

Under appropriate direction the Jews are to be utilized for work in the East in an expedient manner in the course of the final solution. In large (labor) columns, with the sexes separated, Jews capable of work will be moved into these areas as they build roads, during which a large proportion will no doubt drop out through natural reduction. The remnant that eventually remains will require suitable treatment; because it will without doubt represent the most [physically] resistant part, it consists of a natural selection that could, on its release, become the germcell of a new Jewish revival. (Witness the experience of history.)

https://www.ghwk.de/en/wannsee-conference/documents/

On 27.03.1942 Goebbels wrote in his diary:

The Jews are now being pushed out of the General Government, beginning near Lublin, to the East. A pretty barbaric procedure is being applied here, and it is not to be described in any more detail, and not much is left of the Jews themselves. In general one may conclude that 60% of them must be liquidated, while only 40% can be put to work. The former Gauleiter of Vienna [Globocnik], who is carrying out this action, is doing it pretty prudently and with a procedure that doesn't work too conspicuously. The Jews are being punished barbarically, to be sure, but they have fully deserved it. The prophesy that the Führer issued to them on the way, for the eventuality that they started a new world war, is beginning to realise itself in the most terrible manner. One must not allow any sentimentalities to rule in these matters. If we did not defend ourselves against them, the Jews would annihilate us. It is a struggle for life and death between the Aryan race and the Jewish bacillus. No other government and no other regime could muster the strength for a general solution of the question. Here too, the Führer is the persistent pioneer and spokesman of a radical solution, which is demanded by the way things are and thus appears to be unavoidable. Thank God, during the war we now have a whole series of possibilities which were barred to us in peacetime. We must exploit them. The ghettos which are becoming available in the General Government are now being filled with the Jews who are being pushed out of the Reich, and after a certain time the process is then to renew itself here. Jewry has nothing to laugh about...

https://www.hdot.org/evans/#evans_4-3-f

In a 22.05.1943 memo on a meeting between Hans Kammler and the Auschwitz SS we read that "in addition, there has recently been the Solution of the Jewish Question, for which the prerequisite for the accommodation of initially 60,000 inmates had to be made, which within a short time will grow to 100,000" (Die Verfolgung und Ermordung der europäischen Juden durch das nationalsozialistische Deutschland 1933-1945, Band 16, doc. 70). And in the Franke-Gricksch report we read:

The Jews arrive in special trains (goods wagons) towards evening and are taken by a special line to a special area of the camp. There they are unloaded and examined by a medical board in the presence of the Camp Commandant and several SS leaders in the first place to see if they are fit for work. Here anybody who can be integrated into the work process in any way is sent to a special camp. Those with some temporary ailment are sent immediately to the quarantine camp and are brought back to health through a special diet. The basic principle is: keep as many prisoners as possible for labor. The “resettlement action” of the old sort is completely rejected, for it is not permissible to systematically destroy substantial labour capacities.

http://holocaustcontroversies.blogspot.com/2019/08/nazi-document-on-mass-extermination-of.html

The author then describes the murder of the Jews permanently unable to work (which were most of them, by the way - too old, too young, too sick...).

The Nazis were in a war and the Jewish slave labor as an extremely valuable resource, especially towards the end.

And it went towards fulfilling the Final Solution goal, since eventually everyone becomes unfit to work, and slave laborers sooner than later - they would then be disposed of.