Why is buying less automatically better than recycling the mountain of stuff anyone uses to live?

Why is it better to make a smaller mountain of trash rather than figure out what to do with that trash?

The point is that dealing with trash takes time and energy, and if you want to be efficient about it you'd try to make as little trash as possible so you don't need to deal with it later. You might not see much of a benefit on an individual scale, but across an entire city it can make a huge difference.

If you're still not getting it, just compare the EPA's website for Reduce and Reuse versus Recycle

The most effective way to reduce waste is to not create it in the first place. Making a new product emits greenhouse gases that contribute to climate change and requires a lot of materials and energy - raw materials must be extracted from the earth, and the product must be fabricated then transported to wherever it will be sold. As a result, reduction and reuse are the most effective ways you can save natural resources, protect the environment and save money.

Recycling is the process of collecting and processing materials that would otherwise be thrown away as trash and turning them into new products. Recycling can benefit your community, the economy, and the environment. Products should only be recycled if they cannot be reduced or reused. EPA promotes the waste management hierarchy, which ranks various waste management strategies from most to least environmentally preferred. The hierarchy prioritizes source reduction and the reuse of waste materials over recycling.