The basic problem is that the power source is not matched to produce exactly the right voltage and current that the LED takes to put out the light you want. Some conversion is required between the power source and the LED.
Converting DC at one voltage and current to another voltage and current is done most efficiently with pulses. It isn't too hard to smooth out these pulses by the time they get to the LED, but it does take a few extra parts and cost a few extra cents.
In high volume mass market products, those few extra cents add up. People are used to LEDs flashing, so the manufacturers don't feel much push to prevent the flashing. Put another way, way more people are going to buy on price than whether the light flashes a bit. Most people don't even think of looking at the latter, and it's rarely featured, so it's not a consideration in purchasing.
The extra components not only add a little cost, but also weight and they take space. All around, it's just not worth it when selling to the average consumer.