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Why aren't there any straight walled rifle cartridges?
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There are lots of straight walled rifle cartridges in the sense that they have no shoulder, but there are few rifle length cartridges that have no taper the way some pistol cases do, and no modern ones that I know of.
The benefit of the taper is as mentioned, it aids in reducing the force needed to extract the fired case. With a truly straight case after the brass springs back, and you begin pulling the case out of the chamber, the remaining "grip" of the case in the chamber extends the entire length of the case, and is essentially present until you have the case all the way out of the chamber.
With a tapered case, even a slight taper, as soon as the case begins moving backwards, the case is being pulled back, and away from the chamber walls as it moves back, eliminating friction as the smaller end moves through the larger rear section of the chamber.
IF you look at the older rounds made for use in Africa, all are tapered and some to a large degree.
Length matters. As does the system of extraction. And long, perfectly straight cases are the most likely to stick in the chamber.