The books I forget, I assume were not worth remembering. I usually make a point to remember the author's names in those cases, so I can avoid them next time- but I'm not always successful. Lots of times the author's name goes, too.
It's not a compliment if I find someone's work is so completely forgettable. There have been some books I've bought that I forgot completely and re-read years later... and I promptly forgot them again.

Some are near total forgets, like the whole "Amber" series by Roger Zelazny that I picked up in a garage sale a few years back. I can't remember much of that at all. Dimensional travel, maybe- gradual changes like in Piers Anthoy's Mode series? Elves? Still, that's better than a complete brain dump. I know I have it and won't be tempted to get it somewhere else.
Some books I remember quite well, such that rereading them is kind of tedious. Some are old favorites that I re-read every few years or so, and each time I find that I forgot some details that make the re-read fun. There are those I retain a plot outline on, and the near totally forgotton ones and the completely forgotten ones.
I don't think any author can predict how different people are going to react to a story. Some people's brains will find deep meaning in the tale and will record as much of it as possible for future reference. It's all in how your brain prioritizes the data. If it's important to you, or at least tagged that way, you'll remember it. I doubt if people can
choose to be forgettable in their writing, since people are so different in what they find important in a story.