It makes sense in that data is usually written fewer times than read (backups are an exception). Also, if you are paying for the storage of data, then also paying for uploading it to the server would feel like being double charged. Though I guess someone could upload data, then quickly delete it (never download), but there are probably not many uses for this pattern.
Also, while the connection in a datacenter is symmetric, egress would be more loaded than ingress.