Because randonneuring is all about finishing an event in the prescribed amount of time. If the time limit is 40h and you come in at 40h 01m, you don't get qualifying credit for the ride. It won't count towards an S-R medal, or in a year like this, it could keep you from qualifying for Paris-Brest-Paris.

And at ~45g each for those batteries a springtime 400k (5am start) means that you've got 90 minutes of runtime right from the start line. Assume you're an average rider and you'll finish in 21 hours, so you'll roll across the finish line at 2am the following morning. You need 1 battery for every 90 minutes, so at 6pm you turn on the light for safety. That's 8 more hours of runtime, or 5 more batteries. Include the one from the morning and you're up to over 250g in batteries and once they're used up, they're dead weight. Literally. Landfill fodder if you're using non-chargeables, and wasted weight if you're using rechargeables.
Lighter than a dyno, but at 17 hours into a ride in the middle of the dark, and maybe in the rain, how well are you going to do finding those spare batteries and changing them out? (Make sure you're grabbing a fresh one, not a dead one; and that you've put it in right side out.)
What I'm interested in with that light is what the beam pattern looks like compared to an IQ Cyo. 350L is a decent amount of light, about what my L&M Solo puts out with a 13W halogen bulb, but I know the comparison between my Solo and my Cyo, and it's night & day. The lux rating for the Cyo is much higher because of the focused optics it uses.

The biggest thing to remember with most of my dyno vs. battery argument is that it's based largely on randonneuring, not commuting. So overall system weight comes into play very little, especially when talking about the difference of 300g, like the two systems above (including all your batteries for a 400k in the above conditions.) Let's look at other things that some randos carry that are 300g and mostly go unused... I bring a Park mini chain breaker, a section of 5 chain links, and 2 power links. Easily 200g right there. Steel tire levers instead of nylon. Canvas/leather luggage instead of nylon. Stainless steel fenders instead of plastic. The list goes on and on.
The important qualities of a dyno system for rando are reliability, durability, and mechanical drag. Highly reliable, durable hubs with low drag are the holy grail. That's why people go with things like the SON20R (meant for 20" wheels) in a 700c hoop. It's got lower drag than the SON28, but you have to spin it up almost twice as fast for it to work!

I wouldn't go as far to say that new, high power LED dyno lamps are only good in showcase conditions. An old SON E6/z twin setup? Sure. Those got drowned out by an oncoming moped. The Edelux and IQ Cyo both put out phenominal light intensity, and mounted properly will provide all the contrast that you need for most conditions.
Notice, that's "most", not all conditions. Battery systems do have their place. I wouldn't try doing a 24hr offroad race with a dyno system. I don't like how dyno lights look for most packed snow conditions (you have a choice; mount it high and get insufficient, flat illumination, or mount it low and get great contrast with high intensity that reflects back and blinds you.) For the rain and picking out debris/potholes/etc. on high speed descents I prefer my dyno lights. Mounted at the top height of my wheel they have great long-shadow contrast for road hazards. They're low enough that I can aim them very far into the distance without interfering with oncoming traffic (due to the cutoff optics) and that also makes them excellent in rain and fog because they light the road, not the precipitation.