My big cat, Frank, came with gingivitis and needed his teeth brushed. He's a pretty easy-going guy if you play things his way. He had a few back teeth that were especially bad that needed a gum scrub daily. He sat patiently on the kitchen counter while I got out the little toothbrush and the fish-flavoured toothpaste. Wow did he love that toothpaste. He wouldn't tolerate teeth brushing until I'd let him lick most of the toothpaste off of the brush. There was generally enough left behind to help lubricate the brushing, but he wouldn't tolerate it unless I did it his way.
And, years ago my vet pointed out to me that it was nonsense that hard food helped clean cats' teeth for precisely the reason that's been mentioned here. They'll either crunch it once or swallow it whole. Cat's don't chew a whole bunch. And they aren't like dogs who can get a good tooth scrubbing by gnawing on a bone.