A human genome is about three billion letters, and meaningful studies compare many genomes at once. The dataset is far too large for a person to read, scan, or compare by hand, so computer processing is not optional.
Why size alone is not the whole difficulty
Length is only part of it. The alphabet has just four letters, so the text looks almost uniform, and the signal researchers care about is often a single letter that differs from the usual pattern. Finding that one deviation means comparing the same position across very many genomes — a task that is trivial for a computer and impractical for a person.
What three billion letters means in practice
If you read one DNA letter per second without stopping, three billion letters would take about ninety-five years. Reading the same position across a hundred thousand genomes adds another hundred thousand letters to check for that one spot — and a real study checks millions of positions, not one.