cultivating the upright

Training in Chinese martial arts tends to emphasize solo practice at the beginning, especially with the internal styles, like Taiji. Many of the reasons are practical. But I personally think that part of the reason is the influence of Confucianism and the cultivation of character.
Forms practice at first is about learning the movements and postures, then it is about technique and alignment. After this, attention can be put towards building and moving Qi. And it's in this later stage that character - proper bearing and attitude - becomes an important part of forms practice.
If you started with applications and sparring, it'd be easy to become distracted with the endless of details of technique and never make a start in this later stages of learning. And it's not that building character is required to do applications effectively. But if the effective execution of an technique was the only goal, we'd all be better off taking street fighting classes. Extensive solo practice brings the focus back internally and sets the stage for the Confucian ideal of cultivating maturity.