The only time I can remember getting free yarn is when my MIL gave me some yarn and fabrics about 20+ years ago.
I know that I use more than I acquire because every time I put my leftovers away and redo my colors I'm able to consolidate bins.
Once, I mentioned to a coworker that I was using leftover Phentex (extruded olefin fibre) to make a rug for my children’s room. I was inundated with everyone’s leftovers! Result:
Ravelry: JessicaJean's Super Scrappy Rug ca. 1978
Another time, another mother was watching me knit/crochet while our children were taking swimming lessons. She asked me if I would be interested in some unwanted yarn she had. I was, and drove to her house. I hadn’t been expecting her ‘box’ to be the box her washing machine had come in!!! I was fixed for yarn for many months, after spending a full month sorting the tangled mass into individual balls.
Most recently, the daughter of a deceased knitting buddy asked if I would like to take any of her mother’s considerable stash, before she trashed it. I called another knitting buddy, and we loaded both our cars and a guy - friend of the daughter - loaded the rest into his van to bring to my house. I gave away a lot if it, but there’s a ton still to use.
I doubt I’ll ever run out of knittable fibres!