There are some special rules in the Black Mountain Immortal Realm. Some areas are prohibited from flying around, while others have restrictions placed on them. If you were to force your way through, you could very well be targeted by the Immortal Palaces.
On one hand, there are a lot of mineral veins on the way to the southern part of the Black Lands, and most of them are owned by major trading unions, preventing outsiders from barging in as they please. On the other hand, the Black Mountain Immortal Realm's continents are also separated by massive areas that are both poisonous and marshes. Some of the places in those areas are still forbidden zones, just like those in the Wasteland Realm. There are many terrifying demon beasts living in those areas, and there is no lack of beings at the Zenith Heaven level among them.
All of that was stipulated by the Black Mountain Immortal Palace in order to strengthen their control over the cities.