[theorique]

Each person makes his own decision about this, but some prefer to use "they". To each his own.

Everyone has his own reason for choosing a particular pronoun. Some people do it because they think it is less sexist. Some people do it because they want to be part of the in-group, and they see others doing it. Some people are personally offended by the generic "he" and want to refer to both genders so they use the singular "they" or the awkward "he/she".