Why do Christians say we don't have the mental capacity to understand God's motivations?

its a cop out because "they" (the christians) can't explain things like why do bad things happen to good people and vice versa. they are avoiding having to defend their position by referring to someone not available for questioning. it's an intellectually shallow and cowardly answer to give

That usually said as a "bob and weave" of a question about why bad things happen.

Thy just don't want to face the fact that according to thier religious texts this "god" fellow comes off more like a megalomaniacal comic book villain than a benevolent fellow.

Compare the biblical acts and decrees and actions of god and ask yourself is he/she/it/they more like superman and captain America or Darksied or the red skull?

i think its mankind they are refering to with "we".

we as in humanity.