If you carefully look at LED in flashlights, they are blinking at fast rate despite the batteries are DC. It's even more obvious on flashlights with separate R,G,B LED (to allow the control of color) when you move it fast. Why is it so ?
( It's also the case for "power on" indicators in many devices, but here it could just be a low quality DC low voltage from AC 220V. )
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.
The reason LED's are blinking is because they use Pulse Width Modulation (PWM) power the LED. Using PWM, the LED are switched on for a specific period of time and switch off rest of the time. The reason as suggested is Efficiency.
There reason that you are notice the blinking is because the duty cycle in your flashlight might be pretty low. Chances are if the LED is switched on around 200Hz, most humans will not notice blinking. To learn more I suggest investigating Persistence of vision and Flicker fusion threshold
Here is an example of a 50% duty cycle PWM signal
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