There were three people who I really wanted to meet when I was younger. Due to my career I got to meet two of them and speak to a third on the phone. Now I wish I hadn't. Idols are best admired from afar, you really don't want to know what phenomenal douchebags most of these people are in real life.
What do you say when you meet these people anyway, if you don't have actual business with them? "Oh hi, I... er... I really like your stuff"? It's actually a really awkward position, and it's even more awkward for the celeb. Having people come up to you who you don't even know, just because they saw you on a stage or on TV or something, and want to talk to you only for that reason, but essentially having nothing else to actually communicate - that's weird. All you can really say back to them is "thanks for your support" but you don't necessarily have anything apart from that in common, and sometimes you don't know whether to be flattered or just creeped out.