A corporate golf day works when the group arrives as a group, the tee time holds, and nobody spends the nineteenth hole worrying about the drive home. A chartered vehicle handles all three.
Golf outings are fussier logistically than they look. Bags have to travel, tee times are unforgiving, the good clubs sit forty minutes from the office in different directions, and the day traditionally ends at a bar. Split the group across private cars and you get stragglers at the first tee and an awkward conversation at the last.