The metrics are usually about on time departure rather than on time arrival. I have a hunch that OTA is something passengers are more interested in than OTD (I have no evidence to back that up).

Marking the time of departure or arrival is always going to be slightly contentious but it's usually measured by the off-blocks and on-blocks times which, as discussed above, do correlate well to the scheduled block time (not the in-air time).

If the scheduled block time was just the in-air time then what time would you expect to see on the ticket as your departure and arrivals times to help you plan your journey to/from the airport? I'm scheduled to land at 1400 and we're approaching from the east, landing on the northern runway, it's peak hour so expect 10 minutes taxi and 15 minutes holding for a gate.... Oh no, hang on, the wind has changed direction and we're now approaching from the west... It just wouldn't work