"so why aren't they maining morde? he's literally the answer to everyone:
-can lane bot
-can destroy marksmen
-can carry games and decide them"
Technically anyone can be played bottom lane if they're good enough and can combo with their partner.
Sure he can if he can reach them, which is increadibly unlikely as most marksmen are going to be able to kite him without their support's help.
Anyone can if they're fed enough, or if the enemy team is inept enough.
Problem with mordekaiser is that he's an immoble melee without crowd control or sustain. Things marksmen get to build as a bonus when aiming for other stats. For example essence reaver grants plenty of cooldown reduction and mana sustain, so they can spam their cc or mobility to keep away from him. There's an abundance of slows I won't bother listing them.
But here's an example here's their items that give bonus movement speed.
Is he going to build critical hit chance? Doubt it, so there's 3 items already unwanted. Thanks to the rework screwing up how his Q interacts with sheen, trinity force isn't as good as it could be; and it's his only ability with a physical scaling, so the remaining items are quite conditional.
How about non-marksmen movement items.
A few of them are support specific, or geographically reliant, not really helpful unless he's getting dived constantly. The couple of tank ones are nice but obvious and negated if he gets hit by any crowd control. Lichbane like trinity force doesn't synergize as well as he'd hope, but luden's does. His only physical damage is on auto-attacks, but his abilities are relatively low cooldown; of the two physical damage items, black cleaver could be useful if there were other physical damage dealers on the team, otherwise they're inefficient.
You know who should be a good marksmen counter based on their kit? Yasuo.
He's got mobility, crowd control, and denial against ranged attacks. Sure mordekaiser has a shield too, but he has to be able to farm or fight to make it in the first place; yasuo gets one for literally running around, and another of an area denial style.