They flicker at 100Hz due to the rectification (or 120hz in those 60hz countries of course).
Dimmable bulbs use higher frequencies.
One of the bulbs recently burned out, and I picked up a replacement at Menards. Even though it was just a basic Sylvania, the new one clearly has a rectifier circuit as it does not exhibit any flickering that I can detect.
So anecdotally at least, the cheap bulbs without rectifiers seem to be going away from the big box stores (although I’m sure you can still get them with unpronounceable all-caps names from Amazon).
The quick test is to wave your hand quickly in front of the light.
It is done because, like most crappy things in the world, it saves somebody, somewhere, a few cents on the dollar.
Most people would not be able to tell the CRI impact of DC dimming vs PWM. Many do not visibly notice the difference. (I unfortunately do, and you won’t believe how many expensive Mercedes and similar cars flicker).
But high frequency PWM is slightly more difficult and expensive, and DC dimming might need a few more capacitors or inductors… so let’s save a buck there, shall we?