My guess is that they're come individually packaged. Mouser has them labeled and bagged separate. Small Bear has them bagged separate, but not labeled (which is fine because I can figure out what is what from color codes and some DMM testing).
When I run into situations like a big unsorted pil, I start looking for similar color codes. That way, I have all of my 10, 22, 47, 68, etc types in piles. Then I test a few, find out which multiplier color band is which, and put them in the little plastic baggies with white write-on labels from Small Bear. They sell a 100-pack of bags for $4.
Once I've got the resistors in their bags, I group the bags into ohms, k-ohms, and meg-ohms, and put those into larger bags. In other words, I have a big bag marked "k-ohms" and in it are lots of little bags of resistors from 1k through just shy of 1meg.
I find this system to be really helpful. I have a similar setup for caps. I can find all the parts I need for a project in just minutes (most of the time).
I'm usually not this neat and organized, but I was looking for some parts one night, couldn't find what I was looking for, then ordered the parts. A few days later I found that I already had most of what I just ordered. The third time that happened, I got really mad and then got organized.