Not all LEDs are even dimmable, either. For something like streetlamps, which are typically either full off/on, paying extra for brighter dimmables to then run them at a less bright setting doesn't sound an appealing option anyways.

But while we're on the topic of dimmables anyways, quality dimmables should have a switching frequency >1 KHz which is a different kind of flickering than e.g. low quality drivers which flicker at a perceivable rate (50hz/60 hz or double depending on the design).

Flicker isn't intrinsic to LEDs themselves, but rather driver circuits that are efficient and cheap, doubly so for ones that run off low frequency AC mains power. To design a circuit that didn't flicker, first you'd need bulk capacitance to ride out the 100/120Hz AC waveform dips, especially if you didn't want the device to draw peak current around the zero crossing. Then you'd need a high switching frequency and either enough bulk inductance to ride out that off period, or a bunch of bulk capacitance and some wasted power to control the current.

You can modulate the brightness without flicker, you can try it yourself with a lab power supply.

It is just that filtering the output of the regulator is “expensive” as you need a relatively large capacitor. If this is omitted in combination with a low frequency PWM, you will experience flicker.

Not if they are designed halfway reasonably. Any good LED should be driven with a PWM driver at high frequency, with low frequency mains AC (the source of the flicker) completely separated from the LED.

This costs slightly more than alternative methods that use mains directly and don't properly isolate the AC component :(