1. Decompose plant residues and manure to humus.
2. Retain nutrients in humus.
3. Combine nitrogen and carbon to prevent nutrient loss.
4. Suppress disease.
5. Produce plant growth regulators.
6. Develop soil structure, tilth, and water penetration/retention.
7. Clean up chemical residues.
8. Shift soil pH to neutral and keep it there.
9. Search out and retrieve nutrients in distant parts of the soil.
10. Decompose thatch and keep it from returning.
11. Control nitrogen supply to the plants according to need.
12. Pull minerals out of inorganic soil components for plants.
13. Provide the exact chemical nutrients to the plant that the plant has evolved with rather than man's cheapest chemical approximation.
14. Provide exactly the required quantity of nutrients that the plant needs.
15. Provide the nutrients at exactly the right time that the plant needs them.

No chemical can do any of that. To be fair, no single soil microbe can do all of that either. In fact, it could be that it takes 10 or 100 different species, one working right after the other, to do any one item in the above list - sort of like a microbiological assembly line. But at least it's real easy to get all the right microbes. The biology of the soil is very complicated.