The second kid isn't in quite such a rush because that means the purchase of yet another carseat. But alas, you can only squeeze them into that infant carseat for so long.
By the time the third kid comes around, the first infant carseat is kind of trashed and the whole carseat system has been overhauled since your first baby. (Turns out carseats are updated and outdated about as fast as computers these days.) So you find yourself buying a whole spanking new carseat for this third baby who you are determined deserves some new things of their very own. And that new and improved carseat lasts longer than previous models. So this kid gets almost double the time in infant carseat. But, inevitably, the older kids are beginning to outgrow their seats and move into boosters, so you buy some more, spare yourself from totaling how many carseats you've purchased at this point, and shift everyone around.
The fourth kid? Yikes. More than two years later, you find yourself in the midst of this mess: