Why are tardigrades so rare?
From wikipedia &gt; tardigrade

[Tardigrades are] considered cosmopolitan and can be located in regions all over the world.
Tardigrades are often found on lichens and mosses. Other environments are dunes, beaches, soil, and marine or freshwater sediments, where they may occur quite frequently (up to 25,000 animals per litre). Tardigrades, in the case of Echiniscoides wyethi,[20] may be found on barnacles.[21] Tardigrades can be often found by soaking a piece of moss in water.

Tardigrades are, in fact, very common. See the simple video how to observe tardigrades. If the people you've met have rarely seen tardigrades, it is probably because they are not looking for them.