As with many situations with enterprise software vs. roll-your-own solutions (FOSS or otherwise): reliability, responsibility, and liability are the reasons.
Most enterprise software is purchased to do something the company requires, but is not within the company's actual line of business. Payroll, tax calculations, identity verification, etc.
In these cases "cheap" is not very important so long as the solutions are cheap enough relative to the value of what the company's business. This is also why companies routinely contract out vast amounts of work to highly-paid lawyers - paying someone six figures to do work on a deal that's worth 9 figures is a rounding error, and is not a cost worth optimizing.
More importantly, the third party provides two important pieces: responsibility and liability. Docusign is on the hook if they fail to validate the signers' identity, and if anything goes awry Docusign is on the hook for fixing it. These are features, not bugs, to enterprises who need a function performed but really do not want the liability or responsibility around it.
This is similar to why tech companies outsource to cloud services rather than run on their own metal.