I’m not a rabbi, but frozen fruits and vegetables are generally washed, and frozen right in the fields immediately after they are picked, so they don’t have the same chance to develop insect infestations the way fresh vegetables do.
“Raspberries are often infested with tiny insects. Therefore, many leading Kosher agencies discourage their consumption, even year-round. However, frozen raspberries go through a rigoruous cleansing process before they are frozen, which is deemed satisfactory for Kosher standards.”
I use dry parsley, cilantro, and other herbs, but I almost never use the fresh stuff. What’s the difference? The leaves are 1) washed thoroughly and 2) chopped up and dried. So if there were any bugs, it’s a safek if they were still there after washing. It’s a safek if any remianing bugs are whole anymore. And once dried, they crumble and disintegrate. That becomes a s’fek sefaika.
I imagine that freezing facilitates the breaking into pieces any possible bugs left over from a cleansing. Just my guess.