I just think people underestimate the sheer amount of emails that retailers get per day. I think even one of the super small retailers here on SF said they get 200 a day. If you even spend 1-2 (super low side) minutes on each email, you're looking at hours and hours a day of replying to emails.
It's not really an excuse, some retailers with an intense passion for cs pull it off (Epaulet), but it's no wonder the email box can be left unattended at times, while they are super helpful on the phone.