Because they are in Schengen.
The Netherlands and Czechia are now on the list
In which case enforcement will be tricky. You could enter overland from another schengen country and no one would know you were there.
As Switzerland is not in the EU, there are customs posts at many overland borders, some of which (on major roads) are occasionally staffed; but you are correct that people who really want to circumvent any checks could enter via minor roads or even footpaths. I believe that in early 2020 physical barriers were placed over many of the tiny paths crossing the borders, but they probably were easily defeated if one was truly determined.