Because there might be (for example) a time-related correlation. If the file in question were a log file running midnight to midnight, it's possible that there might be less activity at either end of the day than in the middle.

Or (for example), the file might have been previously ordered in some way that means the longer records are at the front and shorter at the end.

Whilst it is possible that a random selection picked across the length of any given file might pick all the longest (or shortest), it is far,far less likely, than likely. That's why you pick a random sample.

It is the whole basis of normal distributions, the 68-95-99.7 rule, and the associated statistics.