Because then it's more likely to have overlap between their startup and the NDA content. And the more overlap, the easier it would be for them to threaten about

Because then it's more likely to have overlap between their startup and the NDA content. And the more overlap, the easier it would be for them to threaten about NDA breach in the future. The most likely scenario: if their startup does really well and becomes popular, you can be sure that this company with the NDA will look and see if they can grab some money with a lawsuit threat.