Why not there is no crime if nobody is watching.
Because people have senses available to them other than just sight, and someone with no skill in sneaking should not be able to make movements to the point of successfully wiping and pocketing items when they're a) the only other person in the room and b) a foot away from the owner.
Also, NPCs should not be that stupid. Your reputation should start to suffer if you're cleaning out the town - seen or not.
"Every time Bob visits a house, all the pillows and kitchenwear go missing.. hmm...". It doesn't take much to put two and two together.
If you want to be a good thief, you should have to invest serious points into it. The same with every skill. In this case, points in sneaking, points in social skills like charm to make you a less likely suspect, etc..
