Happens once over 100 successfull couches, and is barely possible to do intentionally. And even the few cav that manage to do that more or less intentionally put themselves at a great risk doing it. At least it compensates for a tiny part of all the couched lances that got through the enemy's model without registering for some obscure reason.
You got a point, but I'd like to see how NA cav does when confronted to EU inf and ranged though. The rare moments I played cav on NA servers, it felt much easier because people aren't flies using jetpacks over there. The 1st reason I tend not to charge aware people is because even shielders like myself have the required foot acceleration to dodge even the most maneuverable horse, and it gets way worse with ranged, often with >7 ath and extremely light armor. The more predictible the target is, the better for the cav. So it ends up with the best tactic being attacking people that are unaware.
Attacking frontally is extremely problematic in many cases, because you get shot and outranged by 2h and pikes. I guess it can work if you got a lot of cav teammates with you and coordinate your charge though. But with the EU cav pop being so scarce, the usual tactic consists in waiting for the melee fight to begin and attack the flanks/rear of the enemy team.
Its also related to the horses we use. NA top cav use armored horses, which offer the advantage of having okay maneuver at slower speeds, but also resist damage and capable of charging at people in front of them (even if it makes zero sense, in this game people can stop horses trained for war by standing in front of them, this stops with charge damage above ~34). The problem with nimble horses is that if you slow down too much, you get hit more often, and that's a critical problem. This is also why we don't charge often over here. When you have positions guarded by 2 or more archers, you can expect your horse to die before killing anyone, if you are alone.
A couche lance is easily dodged. Even with a 1h you can retaliate with a right swing at the horse's legs. Whenever I'm on foot and someone tries to couchlance me, I do that and usually kill the horse. Now, couching unaware people is a whole different story of course. The risks are there, but they depend on the tactical situation.