This is not really a problem with DAD. It is just you who fails to categorize the movie and instead takes the easy way out. Hypothetically: Why can’t we have a Bondfilm which starts out like FRWL and ends like YOLT/MR? Because you have never seen it before?You can't have an over-the-top film like DAD's second half wants to be with the comparative grittiness of the first half grounding it, and you can't have a true, gritty revenge thriller that the first half somewhat promises with the over-the-top nature that the second half contains.
The idea to begin small and personal, and then continuously add OTT elements until it becomes a Spy-Fi is the best thing about DAD because it has never been done before. Unfortunately, in this case, the story wasn't solid enough to carry the film through that journey. And you could say that they messed up the OTT elements. I liked the Ice palace and the Invisible car, but it is obvious that they didn’t know what to do with them. A complete, unrestrained, Lewis Gilbert ending would’ve been the best thing to wrap it up.