Having been through the approval's procedure for a big USA mobile network operator (MNO), part of the approval's process involved automatically selecting the correct default APN when the modem connects to the network. This can be overridden for special cases, for example if the user needs to use an MNO supplied VPN which requires a special APN.
While I do not know if this is the same for every case, all phones go through an approval process, and if it is a phone approved by the MNO then it is quite likely the MNO will insist that at least a default APN is loaded if none has been supplied by the user. The network that the SIM allows the phone to connect to can be read from the modem without an APN being supplied, so it is quite easy to use this to look up a default APN from list. As phone companies really do not want to change their software for each MNO, maintaining a list makes it easier for the phone manufacturer, the MNO and the end user.