The trees of tropical rainforest have buttress roots because :

Buttress roots are large, wide, plank-like roots that grow from the base of the trunk in many tall tropical rainforest trees [2]. These structures are an evolutionary adaptation to the shallow, nutrient-poor, and often waterlogged soils of the equatorial biome, which prevent the development of deep taproots. Because the majority of nutrients are concentrated in the thin top layer of decomposing organic matter, trees develop lateral root systems [2]. However, these shallow roots cannot independently support the immense height and weight of massive hardwood canopy trees, which can reach up to 60 meters. Buttress roots act as tension elements and mechanical braces, providing the necessary structural stability and anchorage to bear the mechanical load of the heavy trunks and prevent them from toppling over during high winds or heavy rains [2].

Sources

1. [2] https://en.wikipedia.org/wiki/Buttress_root
2. [1] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 3: MAJOR BIOMES > 1. Tropical Evergreen Rainforest Biome > p. 7