Grass seeds require soil to anchor their roots and to access nutrients essential for growth. Unlike mushrooms, which can grow on decaying organic matter, grass needs a substrate that can provide adequate support and nutrients.
When you sprinkle grass seeds on a dead tree, there is likely insufficient soil and nutrients available for the seeds to germinate and grow. The surface of a dead tree is typically too hard and dry, and lacks the organic material needed to sustain grass growth. In contrast, mushrooms can decompose organic matter, drawing nutrients from it without the need for soil, which is why they can thrive on decaying wood.
In essence, while you can provide water and light for the grass seeds, the critical requirement of soil and the right environment for root establishment and nutrient uptake is what prevents grass from growing on a dead tree.