I think it simply is because very few people would describe in text a white couple as a "white couple" and not just as a "couple". In a majority white culture there is no reason to specify skin colour when both are white. It's just a couple.
On the contrary, I think it would be very normal to write "black and white couple" for an interracial couple, and because "black and white" also contains "white" thus those images would show up when you search for "white couple".
This is easy to verify by looking for the word "white" around the images Google returns for "white couple", and they are definitely there - often in the image title itself.
However, if you just search for "couple" then you'll find what you are looking for. At least on my google 9 out of 10 results are a white couple.
If I try the search on flickr.com, "white people" returns black&white pictures of people, while "white couple" returns swans. Meanwhile shutterstock.com returns mostly correct results.
Likewise, since at least around the same time (if not somewhat earlier), Google has also been running OCR on all images it indexes.
Either your theory is wrong, or the other search engines are also manipulating results for political reasons, or they are copying the results from Google.