Why does one have to replace razor blades from manual razors frequently, but an electric razor’s blades are designed to last for such a long time? Is it better material (and why can’t that material be used for manual razor blades?), or does the electric razor sharpen it’s blades automatically somehow?
For the same reason that you have to sharpen a knife but not a pair of scissors.
Electric razors have two cutting surfaces; the blades and the screen/foil/whatever over them. They work like a pair of scissors; cutting the hair between the two surfaces. Lack of sharpness can be overcome by having a little extra “oomph” (which is provided by the electric motor).
Manual razors only have one cutting surface and so have to be able to slice through the hair on their own. A dull blade can’t do this.
Also, an electric razor is less likely to “pull” the hair since it is “pinching” it between the two cutting surfaces. A dull manual razor will pull the hair, making for a much less comfortable shave.