The brain's energy capacity is not unlimited. Making solid, well-founded decisions comes at a cost of energy.
Decision Fatigue is sometimes referred to as "Ego Depletion". And, unfortunately, we need the most help when we're the least likely to get it - end of the day, or when weird circumstances pop up.
Repeated decision making drains our energy reserves more quickly. We start to mentally cut corners for all new decisions, and we don't even realize it. We ignore inputs and signals, start to not make trade offs, and rely on biases.