When babies turn one, a birthday party is essentially an excuse to drink and commiserate about lost sleep. Babies have no friends, which leads parents to believe that their child’s birthday parties will always be fun chat times to hang out with cool adults they already know.
When a baby turns two, they can be entertained, and so a parent may feel pressured to provide entertainment. A singalong might be convened, which makes fun chat times difficult, but is amusing enough to distract a parent from the fact that the party is not, this year, about them. It is also possible, and reasonable, not to hire entertainment, in which case you will mostly hear bored babies crying.
At three, a toddler absolutely requires entertainment. The singalong is now required, and the kids are fully mobile. Children will collide, which prompts crying, but they also have a reliable sense of what cake is, which stops the crying. Parents do not talk much at the third birthday, because they are pursuing three year olds to prevent collisions.