Hi,

It's because where the product is delivered to a customer directly from the vendor's store is called dropshipping.
Please check this blog for further details on Dropshipping in odoo14:

https://www.cybrosys.com/blog/drop-shipping-in-odoo-14

Thank you

In standard, Odoo set up the operation type on "receipt" for Dropship.
Why not "Delivery" ?

Because your inventory directly deliver from vendor that's why.