With American comics, it's not even the shattered continuity, it's that availability is a mess because some of the franchises are so ancient and collectible.
If I want to read through One Piece from the 1997 start, my library probably has/can inter-library loan all 105 volumes, or I can go to mainstream retailers and get any I'm missing without a huge fracas.
If I want to read Batman from the 1940 start, I'd better hope some of the rarer issues come up at auction in the near future AND that I can mortgage my house to afford them.
I'm amazed they never put out a DVD-ROM collection that's "Everything Marvel/DC did prior to, say, 1990, as PDF scans" just so mere mortals have a chance to enjoy the experience of completionism.
I was surprised about that figure of 105 volumes for one piece, so I googled it and it was 109 (probably just what came out since you last heard about it). I was expecting that number to be like a thousand or something.