Because the ratio between the Sequential IOPS and Random IOPS is about 29. Taking into account that part of the data is in RAM, I obtained an "effective" ratio of about 22.
Considering RAM size of 72 GB and your database size of ~225GB, and also the fact that Postgres is the only app running on the server, probably 1/3 of your database resides in memory, so random_page_cost = 22 looks extremely high, probably it completely precludes index usage in your queries.
You should try this setting at least at its default value: random_page_cost =4, and probably go even lower.
Also, effective_cache_size is at least as big as your shared_buffers. Having 72GB RAM teffective_cache_size should be set around 64GB (again considering that Postgres is the only app running on the server).