You make a very valid point. The work process for my organization is quite similar. I've struggled with the exact same issue. Unfortunately, while it is very easy to get data into Dataverse, it is a bit more complicated to work with it once it is there.
I discussed this issue with a Microsoft engineer who works on the dataflow team just last week. Here was his answer:
"This [using Dataverse as a dataflow source] is supported today. I will take an action item to update different documentation articles to make sure the explanation I provide below is more accessible. Here are the details:
Dataverse supports an OData endpoint which you can access with the ODATA connector available in Dataflows. You would need to obtain the organization name, and create a URL like this: https://yourorgname.api.crm.dynamics.com/api/data/v9.1/
We are working with the Dataverse team to include their Dataverse connector in the Dataflow’s “Get Data” screen’s list of connectors, to make this process simpler (i.e. you would not need to construct the URL above yourself) but I don’t have an ETA for that just yet."
Before I knew about this, I developed my own crude and dirty solution. I'm not proud of it, but it got the job done.
If I need to merge incoming data with Dataverse data to perform calculations, I simply connect back to the same external source I had initially used to import the data into Dataverse in the first place. So in effect, I wasn't using the data in Dataverse at all. This worked because my application had the same data in both places.
Sometimes, it isn't that easy, because your dataflow needs data that was created within Dataverse, in opposition to simply referencing data that was imported there from an external source and not transformed along the way. So I devised another even dirtier and cruder solution.
Often the values you need to leverage in dataflows are the ones created by users interacting with Dataverse through Power Apps, or indirectly through Power Automate. In those cases, I created a Power Automate flow to update a (*gasp*) Sharepoint list upon the modification of a relevant record in Dataverse. Then, I could easily retrieve it in my dataflow.
I recommend you do not follow this advice. I am a little ashamed of it actually. But sometimes there's a deadline and your minimum viable product depends on things like this working. So I got it working.
You should do what the Microsoft engineer said, he knows his stuff. Hopefully, the Dataverse team comes through with that Dataverse Dataflow connector so no one ever has to apply this nasty solution of mine.
So I encourage you all to vote on @crmkeeper 's idea here:
https://powerusers.microsoft.com/t5/Power-Apps-Ideas/Dataflow-connectors-should-have-CDS-connector/idi-p/433767
Cheers and keep 'em flowin'
Charles
