You don't get access to the phone line on anything without paying extra money to put a WiFi router there, especially in high-priced locations in city centers. Reusing phone booths eliminates that cost.

The idea sounds cool, but I doubt it's more wishful thinking: it was pretty clear at least six years ago that mobile is the feature and no major landline operator (actually there was only one that matters before it was “split” into two) spent money building or maintaining those phone booths. The # of photo booths is really negligible in many cities.