Exactly, it’s the spatial output (light out at angles) that has the effect with them.
Most of the aftermarket ones are made with 5mm LEDs which have a very very narrow output angle to get the brightness up; most are ~6°. This is where the issues come in; they are bright in certain directions, but not in all as required by DOT. (not that most of us care, but in an accident you may!!!).
There are ways to make “good” replacements, but they will be completely custom and DIY. Nothing off-the-shelf in the aftermarket world will really compare in intensity and spatial output to a typical halogen incandescent bulb.
There are LEDs out there (the ones I listed prior) that will with only the LED (or a couple) produce enough light (At the right angles and intensities) such that it will exceed the minimum requirements needed for a STT (stop/tail/turn) or corner light. They will basically make enough light where you can use a perfectly clear and not fresnelled (no cuts/ridges/etc in the lens) lens and still meet the spec.