Without a compelling incentive to change, people tend to stick to the default suggestion, option or product.

In short: making a decision usually requires effort, and it's easier for them to just accept the default option.

The behavioural science theory of 'nudges' is based on the understanding that people can be encouraged to adopt healthier habits, just by changing default options in their lives.

For example, placing unhealthy snacks further away than healthy ones will lead to less consumption of unhealthy food.

Think of 'nudges' being a way to counter the default bias.