That makes no sense. There's no amount of pushing a replacement part that I don't need that could possibly work. There's nothing to do with it until something breaks. If it does break, then I'm going to search for it. It's not an impulse buy in any way whatsoever.
Same goes for stuff like adapters. If I'm buying an HDMI to DVI adapter it's because I have an old monitor hanging around I want to use with a modern computer, not because I want to improve my cable collection.
Amazon may think you haven't bought a new chair in a while so maybe yours is getting old and worn out. Even if you've recently repaired a leg it's not unreasonable that if you needed a replacement leg for an old chair, you might want (or will soon need) to replace the other legs, or the legs for other chairs you might have. Maybe they just want you thinking about chairs and wondering how soon until you'll need to replace a chair/leg. Maybe they just want you thinking about replacement parts, and the legs are just one example of a replacement part to put the thought in your mind.
I couldn't tell you the number of times that I've bought an adapter that turned out to be trash, or was somehow not what I needed, and had to buy another. Sometimes I even buy 2-3 at a time from different brands/sellers because they're cheap enough, handy to have extras of anyway, and I expect at least one to be garbage that doesn't work at all.
Still, the point is that it doesn't need to make sense, least of all to you. Amazon will use the data it collects how it wants in whatever way amazon thinks will be most beneficial to amazon.
Same goes for stuff like adapters. If I'm buying an HDMI to DVI adapter it's because I have an old monitor hanging around I want to use with a modern computer, not because I want to improve my cable collection.