Higher-resolution-than-necessary scans help with image stabilization. i.e. you can't shift an image half a pixel, so you either don't align it quite right, or you end up blurring it a bit. With too many pixels, you can shift the image just the right amount.
Also, there's a big difference between 35mm negatives and prints. 35mm negatives can actually have 4k worth of detail on them in some circumstances, prints are variable but can occasionally exceed 2k.