Comment by Weves

A few reasons:

1/ No model lock-in / ability to use the ideal model for each use case.

2/ More connectivity. A fuller connector library (contributed to by the open-source community). More built-in tools (similar to the ^).

3/ Customizability and flexibility. If you really need a feature, you can build it rather than waiting months (years?) for your request to go through.

4/ White-labeling. You can make it feel like a product built for/by your company rather than generic.