You might be overestimating the gap in efficiency. For example, last night, I ordered a pizza for pick-up, and the phone call took exactly 31 seconds. Even apps aren't instantaneous, but if they were, you're only talking about saving less than 1-2 minutes per day. In exchange for those time savings, apps introduce a lot of complexity and overhead: service fees, app updates, device storage, logins, behavioral tracking, etc.
Now, that being said, there are a few unique advantages to apps, like being able to order taxis in a foreign country (as you said), or being able to share your GPS coordinates with the taxi.
Now, that being said, there are a few unique advantages to apps, like being able to order taxis in a foreign country (as you said), or being able to share your GPS coordinates with the taxi.