There are varieties of strawberries that are not sour at all, so I suppose it's possible that you simply have limited experience with strawberries. (Well, you probably must, since you don't like them, but maybe that's the reason you don't think they're sour, as opposed to some fundamental difference in how you taste things.)

I actually don't like the taste of purely-sweet strawberries; the slightly-sour ones are better. A very unripe strawberry would taste very sour, but not at all sweet, and its flesh would also be very hard.