I think a big reason (aside from intertia) is that arrow is designed for tables. Json sends a lot more than just that and can support whatever octagonal junitsu squid shaped data you want to fit into it.

Also, a good proportion of web apis are sending pretty small data sizes. On mass there might be an improvement if everything was more efficiently represented, but evaluating on a case by case basis, the data size often isn't the bottleneck.

Because it's a binary format?