look, compare it like this: if fireball wasn't codified in the rules but you tried to recreate it's effects with MMI in play it'd probably go something like this:
"hey GM, i want my wizard to use their magic to shoot a giant explosion of fire at that group of enemies over there"
"uh sure, there's no rules for that but um, cross off a fourth level spell slot and gimme two arcana checks?"
[rolls dice] "26 and 17"
"you create a 10ft radius of fire that deals 2d8 damage to the bandits, it's over 100ft away so your accuracy is slightly off and you miss hitting a few of them"
that's how it feels for martials when they try to do anything not codified and have to rely on GM judgement.