Cat scratching a lot and it's definitely not fleas

Could be she has dry skin. Or allergies. Or it could be fleas and you just haven't actually seen any yet. I'd take her to the vet to have her examined.

She could be allergic to almost anything else in the house besides her food ... cleaners, soap, plant matter on shoes, pollen, mold spore, etc. As AllAbout mentioned, it could be dry skin as well. Even good food doesn't guarantee no skin issues (same as for humans).

Get a vet check, but if you decide to do allergy testing be selective. It's expensive ~ and you can eliminate things at home to try an pinpoint the problem as long as any physical cause is ruled out. Your vet can explain how (most recommend against testing because of the costs).

Check with vet, but when my cat had dry sin we put a little oil in his food. It helped.

Just because she is white does not mean you will see fleas. Unless it is a heavy infestation you may not see them on you or anywhere else. She could be allergic to fleas and if that if the case you typically won't find fleas on them because as soon as the flea bites the cats immediately goes after that spot and gets the flea. If a flea or anything else for that matter bit her it can set off this reaction. Also even though she is on a good food she could still be allergic to it and that could be causing this as well. I would take her to the vet and have them check her out.

My tabby and my boyfriends tabby have plenty of white on them and his other cat is mostly white and when they had fleas you couldnt tell unless you moved their fur around. Unless there was one on the very top of the fur.
But anyway, my female I had lost all her fur once and her skin was flaky. We had no fleas, but somehow she had a couple. And just a couple of bites hurt her and made her itch all over :( Once in a while shes pull fur out when she was stressed(after a move).
It could be so much, or something that can be fixed easily. Id take her to the vet