It's not always the full reason but it is true when I say, "I've found I'm not very happy lending things, nothing against you of course but here's where I got it". Sometimes it's just the nicer way of saying "get your own, cheap-o!". But like most of us, I've had my share of good stuff disappear here and there, or I'm pretty fussy about how I treat books and most people are less kind than I am to them.
I'm big on copyright protection, partly because I started in advertising many decades ago and had to be aware of issues as the secretary to the creative department, also helped some with things like SAG and other contract issues, but also just because I feel it's a reasonable and just thing that I can uphold.
Although I drafted out my own Hazel Hedgehog pattern, because what I was doing is a copy of someone else's work, I bought the pattern on-line. I will always credit it as Hazel Hedgehog by Elizabeth Hartman because that is what it is. The "fair use" part of copyright especially when it pertains to images and art does allow us to make direct lifts from other people's work images but that doesn't make it original and then we are definitely not allowed to use their written text and images as our own. For me, it wasn't right to not credit the artist by not paying for the pattern even though I never would have thought of it on my own but that's why we have lawyers and for someone else that's just fair use.
It gets a bit more problematic for me when dealing with I'm cheap and I buy used books and music. The original author (or artist if music) doesn't get any extra coin, but the person who first bought it got the rights to use it, including to give it so someone else. Still, I like to make sure that I give credit where credit is due when I'm linking a book, or technique or whatever.