Looking at the map of the Amitié cable, maybe they try to keep as much of the cable as possible in deep water. https://www.submarinenetworks.com/en/systems/trans-atlantic/...
The other possibility is that the direct path passes underwater mountainous region with quite variable depths in mid-Atlantic [1], so going up-down or follow the valleys anyway would increase the cable length. The right answer is likely a combination of all these to minimize cable cost, maintenance and latency.
[1] https://earth.google.com/web/@41.47661069,-40.80459324,-4414...
[1] https://upload.wikimedia.org/wikipedia/commons/a/a5/1901_Eas...
Underwater geography is also a major consideration. It's not just the going up and down, it's the risk of landslides, tectonic shifts, volcanoes, currents and icebergs scraping the ocean floor.
On top of that are geopolitics and enviromental concerns.
Of course, it would be possible to have a user in Tokyo connect to Facebook's network somewhere closer to the DC but a) getting onto FB's high quality, low congestion network ASAP will perform yield much better performance than leaving the connection running over the internet where it's subject to congestion and long SSL connection round trips, b) terminating user sessions in POPs allows much finer control over which datacenter users are ultimately sent to. Without this FB would have to rely on DNS changes to steer traffic and they are slow to propagate and crude.