This advice has more to do with how airlines operate than simple logic. Think about their need to fill empty seats at all cost.
If you arrive early for a flight in the morning, you'll find airline personnel who are generally less pressured and free to make decisions in your favor. You'll have fewer people with whom to compete for a free airline upgrade.
But here's a scenario that frequently unfolds late in the process: the coach section is overbooked, but there are empty seats in first class or business class. Airlines are apt to fill all of those better spots before they start bumping.
If you're not around when those decisions are made, you'll miss your opportunity to be considered.