There are three main areas where research informs guidance and design decisions on this type of infant transport. To summarise, there is evidence to support the common practice of rear/inward facing for newborns and switching as they develop.
Newborn vision is limited to short distances in the order of 20-30cm.
Source: Visual Development (babycentre.co.uk, contains references)
They learn to recognise their parents quite early on and until they develop an awareness of object permanence anything they can't see is essentially gone, which we can assume to be fairly distressing. Object permanence was thought to take until age 2 to develop but more recent research suggests babies as young as 4 months understand it.
Young babies are less able to regulate their temperature and a rear facing seat makes it easier for a carer to assess their comfort level.
Why is it common behaviour?
Historically (as much for engineering reasons as anything else) we had Prams and Pushchairs rather than the all-in-one systems we have today. Learned behaviour passed down through generations is equally likely to drive the common practices of today as any specific evidence or guideline.
Similar patterns and practices are observed in sling users, where newborns face inward and are turned to face outwards as they become more interested in the outside world.
Note: This answer does not apply to Child Car Seats, where evidence supports use of a rear facing seat for as long as possible to minimize injury in a collision.