In an effort to cut cost, several cruise lines don't automatically bring out a cake for birthdays anymore. When booking the cruise you need to tell the booking agent you want to celebrate a birthday/anniversary. They will note the record. If the record is noted the waiters will bring out a single cupcake or other single serving of dessert with a candle and sing to the person having the birthday. If you want a larger cake for everyone to share, then you order and pay for it. Just as you did.
Now days so many people are moving away from the fixed dining and sharing tables. The cruise line is adjusting to the peoples wishes with the open dining and arranging smaller tables. You can still request to dine with others in the open seating.