I would guess that the reel change marks, I always knew them as ‘cue dots’ in my projectionist days, are only applied at the end of the reels that are shipped to theaters. DVDs are probably made from master elements that never had the cue dots applied. There may be cases, with older films, where the only existing prints are former theatrical prints and in those cases you’ll still see the cue dots on the DVD or they will have been digitally removed.
FYI information, on prints shown in theaters, the dots appear in the upper right hand corner on four frames, first 10 seconds before the end of the reel and then again when there are only 24 frames (one second) left. These signaled the projectionist when to make the change from one projector to the next. Since most theaters today splice the reels together into one big loop, cue dots are kind of an anachronism.
Prints that have been leased to television stations will often display a lot more cue dots. Sometimes ‘cue squares’, ‘cue X’s’ and ‘cue stars’ at various places around the frame. This is because different stations may have wanted to indicate places for commercials to be inserted. A widely circulated print of a very old movie may have the appearance of being bullet-ridden at the end of nearly every scene. Well, I’m waxing nostalgically. Movies that old are rarely telecast from 35 or 16mm film anymore.