The reason manufacturers do this is because

Most toilets are made of porcelain, a hard and durable material. After firing, the porcelain surface becomes glossy, non-porous, and stain-resistant, making it easy to clean, even when it is white.

When fired at high temperatures, porcelain is white. However, if pigments are added before the glaze dries in the kiln, toilets can be made in various colors. But to save costs and time, instead of adding a coloring step, manufacturers often choose to leave the toilet white.

White often gives us a sense of cleanliness, safety, and purity. This is also why hospitals and bathrooms are often painted white to give the impression of a clean and sterile environment.

White is a neutral color, creating a spacious feel, suitable for any design style and any bathroom size.

Another reason is that dirt and bacteria are more visible on a white surface, encouraging people to clean the toilet more frequently.