Who says no-one thought to look? But apparently only one person on record found them before.

How about you have a peek at the research article, think away the colors from the antibody staining and see if you still consider it strange that it could go undiscovered.

The things in question are appropriately measured on a micrometer scale, fragile to match and tightly tangled up with similar things inside jelly. And needless to say they don't come with helpful colored plastic hulls or developer defined object type annotation for inspection.