The first point is profit. Apple is a very profitable company, which everyone should know. And products that do not meet the profitability standards, even the best, Apple generally will not choose to go public. The research, development and production requirements of curved screens are very high, as is the cost. Therefore, the profit of mobile phones producing curved screens should not reach Apple's standard. Some people may say that they can increase the price! Maybe this trick is still useful for China people, but for countries like the United States and South Korea, many people are still very rational about Apple. Therefore, raising the price will only reduce the sales of mobile phones, and the profit is not up to standard.
The second point is the reason for Apple's strategy. We all know that Apple sometimes uses new technologies to make mobile phones, but sometimes many technologies can't keep up. For example, Android has developed mature fast charging technology, NFC application, infrared remote control and so on. Apple doesn't seem to put these technologies into use, but the reason for doing so is actually Apple's slow and steady strategy. After each function is brought into full play on a machine, new functions are added, so that even if each function plays a more powerful role, curved screens may exist in the future, but the best choice now is definitely a full screen.
The third reason is technology and production capacity. We know that almost all the technologies of full screen are in Samsung's hands. If Apple wants to produce a mobile phone with a curved screen, it has to purchase it from Samsung, and it has to rely on Samsung to make a profit and is also restricted by Samsung. Moreover, the production process of curved screen is complicated, and Samsung has completely used it on its own mobile phone. If we supply Apple again, it is likely to cause insufficient production capacity.