why does morde do so much damage

Adaptive Helm would go a long way to help against it.

And the answer is: because he's a juggernaut. By design, they aren't supposed to be possible to 1v1 because of their stats. That's not really a reasonable class design for a PvP game, but Riot seems to think it is--- despite the fact any time one of them has been strong enough to be played in pros they've absolutely dominated.

Eh the damage isn't that much until later levels. The part that's bullshit is the giant movespeed bonus

His passive is pretty weak on it’s own. Liandry’s and Rylai’s make it strong. If Morde is building these items then focus on either getting some slow resistance and move speed to kite him, or burst him down to his more squishy nature when building around his passive.

they just nerfed his passive in the patch.

adaptive helm or other magic resistance helps vs it.

you shouldnt stand in his passive and fight him its kinda like standing in singed's poison trying to fight him

Simple answer - snowballing. (Riot hasn't done much to stop snowballing.)

This is what I think the situation you are referring to is. Mordekaiser gets a few early kills, and your team has not built any MR - which is rarer recently, given the weaker condition that mages are in (compared with most of the assassins and bruisers). Under these conditions, Mordekaiser can literally steamroll over most lowbie teams.