the motive doesn't matter if you can prove they are doing it.
D'uh
People are incredulous and ask that too, as a way to brush it off; as if to say, in the absence of a motive there could've been no crime.
anon thinks of one example from the past of me answering these type of questions; Thought experiment.
Serial killer leaves bloody knife on stove in kitchen as he ran out?
"But why would he do that?"
Wouldn't he be smarter?
kekek
A deeper problem; these people have no logic.
Another classic is "But it doesn't matter"
For instance regarding 9/11 :
"But we needed to go fight them anyway"
Some of the responses stun you, and it takes a while to recover; meanwhile you can think up an answer. After a while you have a repertoire. It's easier once you've answered the question once.
The same ones will keep coming up.