Scott McNulty, writing for Macworld, explains why:
Devoid of customers, your typical Apple Store is a temple of commerce, lovingly tended by a loyal band of retail employees. But when was the last time you experienced an Apple Store that wasn't packed to the gills with people? Of course, as a company, this is exactly what you want to happen in your store: lots of people coming in to check out your wares. However, a crowded Apple Store (i.e. almost every Apple Store during retail hours) isn't a pleasant place to be if you're on a mission to buy something or have a Genius fix your ailing Mac.
People of all kinds are camped out at the demo machines, doing everything from checking Facebook to filming, editing, and posting videos of themselves rocking out to some sweet tunes. Interested in actually, you know, trying out a machine before you buy one? You'll often have to wait.
Here's the thing, though: All the Apple Store employees are overwhelmed helping the myriad folks browsing and asking questions. When I walk in with a mission to purchase a Lightning cable, I must push my way through the crowds to the corner of the store and grab the cable. Now the fun begins: I need to track down an Apple store employee who isn't engaged with a customer. In the past I've actually abandoned the whole thing and left the store without buying the cable. My ever-clever wife has come up with a solution for this problem though: She simply raises her hand. This is surprisingly effective in summoning an Apple Store employee over the general hubbub of the store.
My local Apple Store is in a mall, and I've never seen it not crowded. I expect that, but at the same time, being there is difficult for me as I don't handle large crowds very well. Luckily, I've always been able to browse and shop without too much hassle.
By the way, McNulty's ideas are good ones; Apple should hire him as their new Retail chief.