It’s because they aren’t installing the correct bulbs. Some dimmers work by cutting the ‘pulse’ that goes to the light early, some of them work by lowering the voltage/current.
When you install a direct LED to one that cuts the pulse, you get flickering. Incandescent bulbs don’t do this because they’re white-hot and don’t change luminosity fast enough for you to notice.
Basically: If they’re flickering – they did it wrong.