Chairs, like wooden dining room chairs, are some of the most dynamically stressed woodworking projects. A bookcase may carry hundreds of pounds of books but you put the books on the shelf and they mostly just stay there. A dining room chair has people sitting down, scooting forward, shifting around, leaning back, standing up etc. so there’s a lot of force moving around trying to bend the frame members and shift the tenons around in their mortises. This often causes the glue, or the wood immediately around it, to fracture under repetitive stress and causes loose joints.

Some woodworkers prefer to use hide glue (or its modern synthetic equivalent) rather than PVA glue specifically because it isn’t as strong, and because the bond can be released with heat. That allows the glue to fail while the wood itself remains intact, and then a chair with a failed joint can be disassembled and repaired. A chair assembled with PVA is likely to break in the middle of a board or dowel and is impossible to disassemble in any intentional way.

Yeah I think the way a lot of chairs are constructed is just a bad use case for glue. Like you said, chairs are under a lot of stress (tension, compression, shear, cleavage, peel- glue can handle some of these well and others not,) there’s a lot of weird ways you can put leverage on the joints, people don’t tend to sit perfectly still so those loads are dynamic and always shifting a bit, and to make it worse they kind of have to be designed to be somewhat lightweight, easy to move around, small enough to fit under a table, etc so there’s always some compromises made and they’re never as overbuilt as they probably should be.

Different kind of construction, but I work in a 911 dispatch center, we have some ridiculously overbuilt chairs that are supposed to be rated for someone to be occupying them 24/7. They cost a ridiculous amount of money and they’re still breaking in new and spectacular ways almost constantly. It’s tough to build a good chair.

There’s also of course issues that can arise from bad surface prep, poor fitment, improper clamping, too little glue, not letting it dry long enough too high/low temperature/humidity/moisture, the wood shrinking/expanding, poorly thought-out joints that don’t have enough surface area or are putting the glue under the wrong kind of stresses, and of course sometimes you’re asking the glue to do something it doesn’t do well, it’s good at gluing wood to wood, but not nearly as good at gluing paint to paint or varnish to varnish.