This is one of the most basic commands DC Pro should have. Make it available and obvious within tools. FLATEEN FLATTEN FLATTEN. Simple merge all layers/ flatten image command. I do not want to go hunting the internet for plug-ins/add-ons/ complicated and convoluted directions on how to do this; nor download some suspect third party program.
What type of file are you trying to flatten? What created it?
If you are talking about Optional Content Groups, then that is something totally different.
What does you mean with "flatten image" ?
Yes, you can save the document as a postscript file. That is the most basic flattening command ever invented since the inception of Adobe Acrobat in the computing industry back in the early 90's. It is also referred to as refrying the document. That said, you may also save as an image, print to PDF, print as image, or use the Export PDF tool, which by the way, all of these actions in Acrobat will result in flattening to a single layer. Every software in the computing world out there (at a minimum ) knows how to do this.
You can also use the Flattener Preview of the Print Production Tool or use the PDF Fixups feature which will allow the user to fine tune a custom profile to control how exactly do you need a document to be flattened in order to preserve image quality and other elements.
Here's a brief article posted by ACP Document Geek:
And here's a link (also taken from the article above) that explains in great detail the why and how the flattening ability that you're frustrated with works or doesn't works the way it does:
Unfortunately, there seems to be no easy one click option, but there are ways around it.