Because the energy in biomass has multiple levels of conversion. Stating with microbes that consume biomass at the particle conversion (base) level followed by subsuquent conversion moving up the food chain.

The higher the trophic level, the less amount of energy is available for those in the higher trophic level. Only about 10% of energy is passed from one tropic level to another. This reduces the amount of energy available and, therefore, the total number of organisms and biomass in each higher trophic level. Biomass shrinks with each trophic level. That is because between 80% and 90% of an organism's energy, or biomass, is lost as heat or waste. A predator consumes only the remaining biomass.The top level of an energy pyramid has the fewest organisms because it has the least amount of energy. Eventually there is not enough energy left to support another trophic level; thus most ecosystems only have four trophic levels.With less energy at higher trophic levels, there are usually fewer organisms as well. Organisms tend to be larger in size at higher trophic levels, but their smaller numbers result in less biomass. Biomass is the total mass of organisms at a trophic level. At the lowest trophic level, the primary producers, there is the least amount of biomass and as the trophic levels increase the biomass increases. This is known as an inverted pyramid. A pyramid such as this one occurs in aquatic ecosystems because the turnover rates of the primary producers are much higher. Therefore, the energy transfer from one trophic level to the next, up the food chain, is like a pyramid; wider at the base and narrower at the top. Because of this inefficiency, there is only enough food for a few top level consumers, but there is lots of food for herbivores lower down on the food chain. Herbivores cannot digest all of the plant material they eat, since they do not have the enzymes, e.g. to digest cellulose. this means that large amounts of plant biomass cannot be broken down and absorbed into the body and undigested material is passed out of the body as faeces. The first reason is that not all energy is transferred equally up the food chain. In fact, only about 10% of the total energy is passed from one trophic level to the next one up. Also, the organisms at the top of the food chain have to expend larger amounts of energy in pursuit of food. Energy decreases as it moves up trophic levels because energy is lost as metabolic heat when the organisms from one trophic level are consumed by organisms from the next level. Trophic level transfer efficiency (TLTE) measures the amount of energy that is transferred between trophic levels.The trophic level that contains the greatest biomass in most ecosystems is the producers. Producers are organisms that are able to make their own food from sunlight or chemicals. Thus, they have access to 100% of the energy available. Secondary and tertiary consumers have to consume a lot more food to support themselves, so there are fewer of them.

make sense