I don't think footballers feel tired the way we feel when we say we're tired. If you asked them on any given day to run 10k, they could. A 50% "tired" pro player would run rings around just about any of us on top form.
It's about their physical condition not being where it should be when they're playing against teams who are at that level. It's about the small mistakes they might make after 70 minutes that cost a goal, not that they need to sit down.
If you think about Olympic athletes, they will spend all year trying to get themselves to peak at the right time. Why are footballers any different? Their bodies will have peak periods, and too many games mean fewer peak periods, which means underperforming.