There are many possible reasons for images not looking so sharp. From the top of my head:

  - When not stopped down, most lenses are not that sharp, especially older ones.
  - Most sensors employ an anti-aliasing (lowpass) filter that blurs the image.
  - The smaller the photosites, the worse the signal-to-noise ratio.
  - Bayer interpolation.
  - Less then optimal post-processing, like aggressive denoising and sharpening with too large of a radius
  - Compression artifacts and color sub-sampling.
  - Analog outputs are usually bandwidth-limited (lowpass filtered) for EMI compliance reasons. Not an issue with an all-digital workflow of course.