Because it was easiest to make it that way.

To be clear, no one (probably) has done any research on this and so no one knows the answer to this question. This answer is to point out a key fact about the evolutionary significance of traits.

Traits, such as the shape of a mango, do not need a unique purpose to have arisen as a product of evolution. If you identify a trait, you can not say that it necessarily fulfills a unique purpose, or even that it is optimal.

Other good evolutionary reasons for a trait to be maintained include:

1) that there's no strong selection for or against the trait so whatever happens to it is not evolutionary important.

2) that we can imagine a better version of this trait but certain barriers (e.g. intermediate fitness, resource availability) have prevented it from arising.

3) that we can imagine a better version of this trait but it would require the loss of a different, more beneficial trait.