Figaro, this is the problem I run into ALL THE TIME. I get really frustrated about it, too. It seems that my LYS only will carry one brand of needles, which is frustrating. Depending on what I'm knitting and the yarn I'm using, sometimes I like the Clover bamboos, sometimes I like Addi's. Also, even my LYS didn't have all sizes of DPN's which
really frustrated me--if you can't go to a LYS for knitting needles, where else can you go???
I think the Michael's, Hobby Lobby's, and JoAnn fabrics-type chain stores don't want to carry everything in stock for knitters (all size needles and more "gourmet" yarns) because they are stores that cater to
many crafting hobbies, and have to make sure they carry a little bit of this and a little bit of that. I like to think of them as the corner convenience store--they carry the basic things they think knitters will need (like the "bread" and "milk", so to speak), but tend not to carry the more designer yarns and various size/types of needles. Plus, they have to make sure that whatever they stock will move out of the store and be sold within a reasonable amount of time. Maybe the Red Heart, Caron, and Lion Brand yarns move out really quick, but say maybe a more designer and pricey yarn would be sitting around too long. Then sadly, there is a theft factor. These stores are pretty big and don't have the capability to keep an eye on things as well as a LYS would (because they tend to be smaller than one of these bigger chain stores), so they may not want to keep yarns or supplies around that could get shoplifted easily.
You may want to see if you can special order things that you don't find on the shelves. Some of the stores are willing to do this if you ask. Some of the stores on line can order things too and will ship it to you; occasionally, JoAnn's has a special with free shipping and no minimum purchase and they have a variety of knitting needles sizes on line that you don't find in the stores. I think the chain stores will eventually wind up carrying more sizes and varieties of yarn because knitting is continually gaining more and more popularity. I can remember when stores NEVER carried fancier beads, and now, Michaels and Hobby Lobby carry a wide variety of fancy beads, even semi-precious. This to me says that it's only a matter of time before they begin to carry more for us knitters, too. And if we don't tell them that there are things we'd like to see them carry, how will they know that there are things that we need but they don't carry?

These stores want to compete against each other, and I think generally, they will try to accomodate the consumer.
I understand about the spending thing with husbands, too. I feel so badly sometimes because it's the same thing with me--I'll have spend x amount on knitting stuff, but it didn't include everything I needed, and I feel bad telling my husband, "Oh, I know I just spent $x, but I
still need..." I feel ungrateful and kind of mean asking for more money so I can get all of my knitting supplies :(