Why do iluminating LEDs flicker?
Because they are built down to a price. Reducing flicker costs money.

Because a large part of the population don't notice the flicker, and buy products based on price, not performance. There are no standards AFAIK anywhere in world requiring manufacturers to label products with how much they flicker.

The mains delivers very little energy around the zero crossings when the voltage is low. If you want a LED to emit a steady light, then you need to store energy when the mains voltage is high, and release it to the LED when the input voltage is low. This requires an extra component, called a capacitor. You can save a few pence by leaving it out.

If you omit the energy storage capacitor, then the LED goes dark 100 or 120 times per second when the mains voltage is around the zero crossings.

I unfortunately have very fast eyes. I rejoiced once CRTs were replaced by LCDs, only to weep again as I realised that almost all LED light manufacturers were cheap, and produced stuff that flickered. I am now building a little portable 20W mains-waveform inverter, so I can power-up lights in the shop before I buy. Some brands are better than others. Exterior floodlights seem to be the worst.