A good sellers agent will know better than you how to prep your home for sale, how to market it, where to find qualified leads, what price to set it to, where to get contractors to make repairs, what kind of repairs are worth making, and when you get to the deal phase, how to negotiate and what paperwork is necessary, and how to deal with contingencies.
A good buyers agent will have deep knowledge of the neighborhood you're looking at (or can take your requirements and find you a good one), will have strong knowledge of the local schools, local demographics, local issues that can affect homeowners, what to look for in a home that could be a problem after purchase, red flags, and can help you negotiate the best deal and extract the most concessions from a seller.
But in both cases, I don't see why the value they provide changes based on the value of the home. Some of them might change based on the size of the home or property, but not the home value.
I don't see why we don't push for fixed fee based services, perhaps with an adjustment for local cost of living and size of the home/property? I know the Realtor cartel doesn't want that, but how can we citizens make that change happen?