The trouble is, all that food is pre-cooked, and sitting rotting in the heat with flies everywhere. The turnover in food whilst high at peak, is not sufficient to ensure freshness. Then, look at the hygeine..... how are plates etc washed and dried.
If you have a strong constitution and are acclimatised to the bugs, you may get away with it for a while. Inevitably you will suffer at some point, as do the locals..... the penalty you pay for eating real Thai food. If you see it freshly cooked in a wok it is safer.