There used to be thou, which was a singular form of you. However, thou also implied you were talking to someone at or lower that you were. Eventually, it became seen as rude to call someone thou, so its usage dropped in favor of a uniform you.
A singular they fits this role, as the gender isn’t defined enough to use he or she and the use of it would be seen as an insult.
This is typical for the English language.
There used to be thou, which was a singular form of you. However, thou also implied you were talking to someone at or lower that you were. Eventually, it became seen as rude to call someone thou, so its usage dropped in favor of a uniform you.
A singular they fits this role, as the gender isn’t defined enough to use he or she and the use of it would be seen as an insult.