Open source software doesn't generally have a company behind it that you can obtain support from via a contract. Some do, but a small, but dedicated library that your entire company relies on? Probably not.

Additionally, there's some perception that paying for software results in a better product than paying zero, which is an intuition from the adage "you pay for what you get". Programmers and users of open source software generally believe the opposite, but executives and middle managers are in a completely different headspace from the workers that produce and use these products.

There is one aspect of that which is true: If upstream breaks your product, you have to figure it out. You can't (or at least shouldn't) just yell at some company upstream and hope they unbreak things. So, the support costs become the company's costs, and who knows how much those costs actually are if you aren't ready to track such thing?