New species are collected all the time. But unless they are big and spectacular, resulting in instant attention, they languish in museum collections and elsewhere for years or more - on average, about two decades. Why? Money. Expertise. Scientists like Terry Irwin may collect new beetles at a rate of thousands a year, but only an estimated 200 specialists are qualified to determine their taxonomic affinities and write descriptions on them. An item I saw in the 1990s said 600 new mollusks were being described each year, but two-third of those were from museum specimens rather than being in the field.
The trouble with this is that we can't make intelligent conservation decisions without current knowledge of what's been discovered. As Benoit Fontaine of Muséum National d'Histoire Naturelle in Paris says, "Species new to science are almost never recognized as such in the field...it often happens that we describe species which were collected alive decades ago and which can be extinct now -- just as astronomers study the light of stars which do not exist anymore."
Cryptozoologists often wonder what exists in old museum collecting rooms, maybe crumbling to dust. So, it turns out, do biologists.