I disagree. I don’t see why that isn’t a sufficient answer. Yes, in many cases you could get away with it, and in fact many people as you say do; however, you may not always, and once you are caught being immoral, there are consequences, such as people no longer trusting you, and therefore people have to factor that into their decision making process. It’s a cost-benefit analysis that people make, and the costs are the punishments and consequences you might endure compared to the relatively benefit you might gain. Look no further than why most people jaywalk but few people steal from others.
As it relates to Plato’s thought of experiment, yes, you don’t have a “real” reason to be moral if no one is looking or likely to find out. However, this isn’t something to worry about because human beings are social creatures, and so we have to live in communities of others.
I think you are starting from the erroneous references in presuming that such a “real” reason should exist in the first place, given that there is little evidence for it. If we can find a rational scientific, read “evolutionary”, reason why you shouldn’t steal from someone, because the reward isn’t worth the risk in the long run, why go searching for supposed objective reasons beyond that.