Front lights are for you to see things, and for other people to see your light. The problem with really bright lights is that they tend to have a tightly focussed beam. That is great if you are using it as a spot light to look at a specific object. Its also good for marketing, when people pick up a small light in a shop, shine it in their eyes and thing "Ooh thats bright". Its not great if its pointing in the wrong direction. Its pretty bad if it ends up pointing directly at someone so they can't see anything else for a bit.
For a bike light, its not that useful to be able to see things from a great distance. Its much better to be able to see things from a wider angle, such as a driver looking to come out of a side road.
It a particular problem for a rear light, where you only want your light to be seen by someone else. I would much rather that I would be seen by someone coming at an angle - perhaps part way around a corner - than being seen by someone half a mile away. If a driver is directly behind you, then a reflector will also be visible.