Why can even a uniform layer of food develop hot and cold spots in a microwave?

Answer: Reflected waves create regions with stronger and weaker electric fields

Microwaves can heat only the thickest parts of food — Microwaves do not heat only thick parts, and thickness cannot explain hot spots in a uniform layer. Food shape and composition can matter too, but the oven also has an uneven microwave field.

The turntable blocks the waves whenever it rotates — The turntable does not repeatedly switch off or block the microwave field. Moving food through different regions can make heating more even. The hot-and-cold pattern comes partly from interference between waves reflected inside the oven.

Reflected waves create regions with stronger and weaker electric fields ✓ — Microwaves reflect from the metal walls and overlap, creating a pattern of stronger and weaker electric fields. Food absorbs energy at different rates in those regions, even if its thickness is uniform. Rotation and stirring help average out the exposure.