The challenge of syncing from stubborn SaaS tools to your data warehouse / database I suspect is different than syncing data from your data warehouse / database back to SaaS tools. Specifically, reverse ETL has to incorporate more context from the business I guess so the data that lands in the 3rd party tools is actually solid.
Once you have customers and a good network of integrations with a large number of tools, I suspect it's easier to just buy that company than build it all yourself?