Most of the documents people e-sign are related to business deals (contracts, NDAs, etc) or HR (offer letters) which are such high dollar value transactions that the cost of e-signing is negligible.
Your use case unfortunately is just not worth it to them in comparison.
Precisely. I first used Docusign when signing an offer for a condo. The subscription fee is handled by the RE agency. The commissions on a single sale pay for a 1-user license probably 1000 times over.
One may as well ask "Why does Adobe charge $30/mo for Photoshop?"
Your use case unfortunately is just not worth it to them in comparison.