The reason bodybuilders eat rice and not pasta is because rice is a great source of carbohydrates.
The carbohydrates in rice help the body bodybuilders replenish their glycogen in their muscles which is depleted during intense workouts.
White rice also ranks high on the glycemic index.
The glycemic index measures how foods affect insulin levels and blood sugar levels.
Rice is also a good source of protein.
When rice is combined with chicken, the rice provides the bodybuilders a great mix of carbohydrates and protein which helps to build muscle.
Chicken is a good source of protein which is also very important for muscle growth, immune function and muscle repair.
Rice and chicken is also a cheap and clean meal and very filling.
The rice is also smaller, so the rice digests faster than pasta and noodles and then it can go right to your muscles easier to get bigger.
Usually pasta has more calories per gram than rice and may have more fat.
Bodybuilders basically choose rice over pasta, especially white rice, as a staple carbohydrate source because of it's digestibility, calorie density, convenience for meal prep, and the ability to pair the rice well with lean proteins to support muscle growth, despite it's relatively low nutritional value compared to other grains.