It's because inf is in the archers way and sometimes archers block other archers. It's realistic and somehow different than in Warband. Just use a different setup, don't expect arrows to go through your soldiers heads without damage. So they don't shoot at all.
To be honest:AI could be a little bit smarter at that point. Moving some meters out of formation to find a spot to shoot from shouldn't be a problem or big deal. Because they stick in their formation they often block themselves or get blocked by other troops nearby.
Getting out of formation is how you get Gleive'd. What also works is spreading the archers (F3-F3), and putting them in an elevated position. I'm yet to find a map, even in Khuzait steppes, that doesn't have many better and worse hills for this, often you just have to back off a little to get troops in neigh unassailable position.
Crossbowmen seem particularly bad at dealing with this.Another aspect is, AI often turns slowly. This results in archers never shooting and always turning, getting slaughtered by horse archers. It's the same problem in tournaments. AI turns slowly -> easy win for experienced player. Just circle around them.
BTW, I've confirmed - mounted archers shoot a lot more to the front than to the back. Because if their shot gets out of possible angle, it shoots straight - so catching the tail of enemy archers is very doable even if you can't get on their right flank. Just not with charge or straught up follow - sometimes I tell them to disengage or go to a far away point - just so they re-align, and you can have then follow for more flanking.