Why?

Libraries doesn't make code looking more professional. It's a way to let many projects share a common set of functionality.

But if the code is stable (which it really should be, or it's questionable if it really is suitable for a library), then it doesn't matter so much if the source code itself is duplicated for multiple projects.

In many situations, only code duplication is really acceptable. And the reason is that embedded projects can live for 10 years or more, and company A can sell the product (i.e. ownership of source code etc) or license the source code to another company. Then it is good that the project is complete. A library means a huge cost, if one of the functions is later found to need to be changed and the full functionality of the library has to be recreated because the access to the source code has been lost.

Even without company mergers, a number of companies every year do manage to get parts of their source code lost, and ends up with important projects that can't be completely rebuilt.

Edit: One more thing - if you want to debug, you need access to debug information. And the corresponding source code.