Thatch build-up depends on the type of lawn grown; fescue does not form thatch (at least not to a substantial degree), but zoysia does. Thatch is the pile-up of soil-level grass growth, not so much (as we often think of it) accumulated dead foliage or clippings. Thatch-removal machines tend to cut through most of it; what's pulled-up tends to be some dead grass blades and debris, but much of it can't be pulled-up without causing more lawn damage, so it's sliced apart instead.

Since we force lawn grasses into frequent re-growth as a response to regular mowing, fertilization, and irrigation, this speeds-up the formation of thatch and debris build-up. Mowing imitates natural grazing damage, in that the grass' response is regrowth, but in nature this frequency of leaf loss would not occur on the same plant multiple times per growing season; hence, the support from fertilizer and water. The composting of plant debris in-place due to microbe and invertebrate activity does happen, but can't keep up with the amount being deposited due to this boost in growth. In wild grass-dominated habitats (meadows, grasslands, prairie), thatch doesn't build-up too much because seasonal fires help recycle the material by turning it to ash every so often. (Plus, many of those grasses are bunch-forming, not spreading grasses that are the main culprits for forming thatch.)

Grass clippings left on the lawn (once processed into finer pieces from a mulching-type mower) do help build-up the soil's organic matter for the benefit of root health, though yes, over-developed thatch can prevent this material from reaching the soil surface where it needs to be.

Miri