If you have a stable global long-term API key for a global API that you use without regards to your users' tenancy on that platform, then absolutely - just put it in your 12-factor env or other secret storage, and start developing!
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.
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.