Earlier quoted context omitted.
My friend built a house. I wanted a house. It was easier to take the lumber from his house than cut down trees and mine iron for nails. Eating another is no different. Different life evolves to take a niche item (like grass or minerals) and turn it into something useful to them (muscle and fat) that is also useful to others.
i don't think that analogy holds up because it seems to ignore that there are other methods of obtaining energy. and your last statement isn't getting to why. it's just what happens.
every living organism has some way of perpetuating itself, either by growing indefinitely or by making copies of itself. there's no reason "why", other than that organisms that can't do this get filtered out very early in the game. this implies having some sort of "strategy" for traveling up the energy gradient.
short of somehow evolving the biological capability to fuse/split atoms, all the usable energy on earth comes from the sun. so the first link in the chain has to be something like photosynthesis, where you collect and store energy directly from the sun (ie, "building a house"). but in doing this, you change the gradient, creating an opportunity for other organisms take your stored energy for themselves. in some sense, it's much "easier" for a rabbit to eat a plant in one minute than it was for the plant to collect all that energy over its entire life. there's only so much solar radiation per square meter, which limits a plants energy budget. a rabbit can eat many plants in a day, effectively multiplying the area of the solar radiation it can capture (and spend on acquiring even more). but then the rabbit creates an opportunity for a predator to harvest its stored energy and the cycle continues...
it seems like you're maybe asking "why can't all life just cooperate"? this would probably look like a planet full of plants (or a similar lifeform at a different scale). the problem is that this leaves a huge opening for the first "defector"; it's an unstable equilibrium. life does not evolve to leave opportunities on the table.