I wouldn't say that's an accurate generalization for all plants.
But to the extent it's true, my theory is because most plants are by nature immobile. Sunlight is everywhere so moving much would be a waste of energy. So plants grow, shrink, open, close, turn, hibernate, but they can not move to a more sheltered location. They must stay where their roots are and face whatever hostile conditions they might encounter (winds, cold, snow, rain, predators, etc.). Mobile, food-hunting animals, on the other hand tend to have fewer natural defenses against harsh environmental conditions and predators. So they rely on shelters (found or built) for protection instead. Mammals adapted to a particular type of harsh conditions are less likely to seek shelter from them. (eg American Bison, for example, are adapted to handle cold and snow.) Cold-blooded reptiles may use shelters but at times environment may compel them to leave the shelter in order to bask in the sun to increase their body temperature.
It's also partly a semantic issue. "Environment" tends to imply a background that contains mobile actors. Stationary plants thus are environment, moreso than animals.
But to the extent it's true, my theory is because most plants are by nature immobile. Sunlight is everywhere so moving much would be a waste of energy. So plants grow, shrink, open, close, turn, hibernate, but they can not move to a more sheltered location. They must stay where their roots are and face whatever hostile conditions they might encounter (winds, cold, snow, rain, predators, etc.). Mobile, food-hunting animals, on the other hand tend to have fewer natural defenses against harsh environmental conditions and predators. So they rely on shelters (found or built) for protection instead. Mammals adapted to a particular type of harsh conditions are less likely to seek shelter from them. (eg American Bison, for example, are adapted to handle cold and snow.) Cold-blooded reptiles may use shelters but at times environment may compel them to leave the shelter in order to bask in the sun to increase their body temperature.
It's also partly a semantic issue. "Environment" tends to imply a background that contains mobile actors. Stationary plants thus are environment, moreso than animals.