Why are we supposed to feed one brand?

Hello! I have a 4.5 month old kitten who is very very picky. I’ve figured out a few brands/types of wet food she likes and originally I was feeding her a variety of all of them. Never had any problems with her acting off or feeling bad. Typically I would feed her a couple brands a day.

When she was 8 weeks old my vet told me to only feed her one brand of food, which I listened to and switched to only one brand. Fast forward to now and she is totally sick of only eating the one brand.

She seemed to like the variety before and I never noticed an upset tummy, but all of the packages of food say to switch over a week long period.

Can I feed her multiple brands per day?