Because if you're trying to view an interlaced video on a non-interlaced screen (like a computer monitor), the fields won't match up; one will be slightly offset if there's any movement. It'll look like you're looking through venetian blinds.
In order to get rid of the interlacing, you have to use some technique to get rid of it. All of the techniques cause resolution to be lost. The most common one is to strip every other line and just double the remaining ones. As you can see, you'd lose half of your resolution. Another way is to blend the two fields, but this makes it blurry and you lose detail. There's no perfect way around it, other than recording the video in progressive scan mode.
