Because bandwidth costs money would be my guess. You upload a nice large 6mb file and they reformat it and you download it thats 12mb of bandwidth, factor in a thousand visits a day ( a small-ish site) each only editing the 1 image and you're looking at 12 gigs a day 360gb of traffic a month. and thats only 1 image. Double the visitors and each doing 3 images and your into couple of terabytes of data. and still its only a low traffic site. ramp that up to a large site and it gets off the scale in terms of data traffic.
So it makes sense for them to reduce the file size down as much as possible, the vast majority of their users are editing the images for screen display and dont really need high DPI images. Another reason will be processor load, the larger images take more time to process and reducing their size takes the strain off the server.
or at least thats my guess.
If you really want to keep the file size do like someone else suggested, grab a copy of gimp it's free, or lightroom its not to expensive and use the offline solution. For an online solution try flickr, it doesnt reduce the size of the image as far as I remember and now with the free terabyte of storage is quite an attractive offering.