Why Help Shows Up When You Don’t Need It.. and Disappears When You Do?

Why is it that in shops where you don’t need assistance.. like sunglass stores.. staff are on you the moment you walk in, asking what you’re after, even though they only sell one thing? Yet in places where you do need help.. like shoe shops.. no one’s around and the staff seem to vanish.

I’m in the UK in West London, is this just an area thing? Or is it the same everywhere?