The repairs at central facility have fixed costs (shipping the device, tracking, paperwork), the not-repairs at customer center have fixed costs too. Carrying out simple repairs on site would be much more more efficient if they could be done on site. There is (albeit not trivially measurable) opportunity cost associated with customer inability to drop device for few hours and have it repaired.
Of course there is a delicate balance between various costs (design, manufacturing, repair, lost product upgrade sales, lost overall sales) and increased repairability may mean decreased profits, but I am arguing the main point.