Main reason: Everyone face unique problem and unique problems need unique solutions so having access to APIs let users to create their custom solution as per needs without waiting for you to craft for them and take ages,

Having access to API does not harm your business as ultimately users are in your platform.

like I see many people asking to integrate a new payment gateway or courier services so if u give them APIs then they can integrate themself.

Look at Shopify they let their users to use custom apps and integrate APIs even the top ecommerce company knows they can't create unique solutions for their unique customers

Even your competitor Instamojo provide APIs both Payment gateway and their online shop