1. Going 'possed up' means the one or two security guards on duty are much less likely to detain you, and that the detainee can be freed by force if requiredI've no idea how much stock Oxford Street loses to shoplifters at this time of year (Bill Quango might know) but clearly it's still profitable. And Oh yes, there's a point number six;
2. The huge crowds provide good cover, especially if six or eight of you hit a store at the same time
3. Security guards aren't heroes. They know you carry knives, and they know their hi-vis vests aren't made of Kevlar.
4. Most police are still on leave, and those on duty won't even respond to a shoplifting incident
5. Even the kind of legitimate shoppers who use the same shops you target are unlikely to be from that cohort of society who will intervene to stop you.
6. Don't target the same shop at the same time as a rival shoplifting gang ...