Like New Judge says, as they can often get away with it. I worked in a supermarket (albeit many years ago) and have seen quite a few incidents of shoplifting from shops in the city centre here. A lot of the attempts I've seen are pretty blatant.
I've also seen plenty of attempts to stop shoplifters getting away. I commute via an area of town with a pretty high crime level and lots of small shops/coffee shops which have issues with shoplifters/theft.
Even if they are spotted, there is then the matter of actually catching then restraining them once they leave the premises. I've seen some really nasty confrontations with staff/security, how much do you put staff at risk, especially as you never know what they might have on them?
It sometimes seems to be more of a try and get the items stolen back, if possible, or at least know what was taken for the paperwork. Ban them if possible.
Even if they are caught, there isn't much chance of the police coming out unless it involves, say a very high value item(s), someone particularly prolific or the use of threats or violence.