We'll wait and see if the resident experts will offer some details, but allow me to give it a try in the mean time.
Kolaris 说:
Weren't feudal Japanese cavalry usually a ranged force?
Not in this period, in fact the bow archery fell out of fashion even earlier. Yes, the early mounted samurai were more akin to horse archers but by the sengoku period the samurai had traded his bow for a short spear and his armor had become tailored for fighting on foot as well as in the saddle.
It was also generally safer to avoid any stray projectiles (or enthusiastic head hunters) by sniping from friendly lines.
Kolaris 说:
I'd heard that Takeda Shingen was notable for moving to more Yari-armed cavalry, possibly due to the superior breed of horse available to him.
The thing about Takeda is that they had access to some of the finest horses in Japan, so it wasn't only about tactics but also that many different clans simply didn't have the horses to implement them. And even when they did, bear in mind that local, period horses were significantly smaller/weaker then what you may imagine. And that those particular nags had to carry a fully armored warrior.
Now additionally try to imagine how effective those mounted archers would really be trying to skirmish with (many times their number) enemy gunmen and archers hiding behind shields.
Kolaris 说:
I can't think of a reason all cavalry in the mod is melee, and the tradition of mounted archery is completely absent.
I know what you mean.
Even if not 100% accurate M&B is not M&B without hunting down horse archers stuck in a riverbed.

I'd personally love some mounted gunners even though they would be a pain to balance properly.
And it would be nice not having to mod them in manually between updates...