But if, for every integrated service, you need to build out a custom URL to receive OAuth tokens after an authorization flow, and a database to securely store them, and a reliable background task to periodically refresh them... then you're doing a ton of work before you can even access a Salesforce or Quickbooks object on behalf of your clients. Having a service that can orchestrate all of this can make it as easy to work with as a stable API key, and then you just need to do the work.
Things don’t need to work that well to be sold. One of my employers has entire integrations that don’t work and we still sell them to the business types successfully. They then file a bug ticket and wait months until we get to it, them paying fees all the way.
Building integrations comes with specific challenges (security, ETL, etc.), which are usually not the core competency of SaaS teams.
We often see that problems related to building integrations are underestimated and can spiral in terms of engineering costs. Authorization in itself is complex, data synchronization even more so!