Greedy by earliest start time: always pick the activity that starts earliest.
Example: (0,10), (1,2), (3,4). Greedy picks (0,10) first. Now (1,2) and (3,4) both conflict with (0,10). Greedy gets only 1 activity. But the optimal is (1,2), (3,4) for 2 activities. Starting early does not help if the activity runs too long. It blocks better options.
The problem: you commit to a long activity before seeing what it blocks. This greedy strategy fails because start time does not capture opportunity cost.