Not allowing third-party libraries in your software package has many advantages:

• You are not worried (your reputation is not tarnished) about other people’s bugs which are out of your control
• Releasing, deploying, and versioning become simpler to a lot simpler
• Debugging becomes simpler to a lot simpler
• Backward compatibility comes under your control
• You have a free hand to revise your design and code

It also has some disadvantages:

• There might be functionalities that are hard/time-consuming to implement that are already there
• If you find a battle-tested library, the debugging is already done for you
• There might be industry-wide standards/trends that you want to follow by using a reputed library