Why are tickets booked individually?

The reason why itineraries are booked individually is due to the fact that each ticket is reserved individually

When you are picking trip itineraries, you may find that your departure itinerary is with one carrier and that the return itinerary is with another carrier.

This is to provide you with the utmost convenience and adapt the trip to your time frame of departure date.

To avoid any payment discrepancies, once the payment total is authorized, the payment charge for each ticket will settle separately so that all transactions are correctly processed with each carrier on your trip. Keep in mind that if you are booking a round-trip itinerary, each ticket will be booked individually, and will appear as two separate charges on your bank statement.