Many companies create re-usable code whereby it would be profitable to share across business units. If the bureaucratic friction within is too expensive, it may make more sense to use external code.
For example, let’s say the company sells PDF conversion software. With that sale, there must be a licensing agreement that documents restrictions for the purchaser, as well as other legal constraints.
If another product includes the same PDF conversion software, say for a newspaper website production package, all of the prior licensing must be rolled up into that offering as well. This all seems simple and straightforward, until it gets complicated.
As more and more business units or products wish to share home-grown technologies, the licensing structures can get complex. Maybe not today, but as this sharing expands & contracts over different products or business units over months and years, the amount of practices, side-deals, exceptions and policy changes make for a special quagmire.
Silly of you to think that all you needed to do was include the conversion software and be home free! Compare these burdens with the well-documented, published, legal document that accompanies the external software.