All wedding cakes are customized cakes. Most people don't buy an off-the-shelf generic cake for a wedding. A wedding cake is understood to be made especially for the occasion.
It was not even a question in the lawsuit that a pre-made cake would have been an acceptable substitute.
I said "most people" because I'm sure there are people who have less formal, low-budget weddings where they grab any old cake from the grocery store. That is not what is understood when someone says "wedding cake".