Radios with fixed, pre-set frequencies (Walter Dittel FSG16). Why?

I am in possession of a Walter Dittel FSG16 and I sometimes fly club owned gliders which also carry the FSG16.
To my absolute surprise I found out after several years of using them that they only support 12 pre-selected channels/frequencies which can apparently only be set by an authorized technician (who needs to open the units to set the frequencies). This was a what-the-hell moment.

The first surprise is that I didn't notice this before but that's because I've only been flying "around the patch" where only one frequency has been required and I've never bothered to change it.

The second and bigger surprise is that someone would design and manufacture such a "hard coded" solution.
This seriously limits where I can fly and land as some of the nearby airspace and airports are not in the list of 12 preselect channels. I could get the pre-set channels updated but then if I travel with the glider I'm back in the same situation. I guess a modern unit is in order even though these old units are solid and reliable.

Does someone maybe know why these radios where designed to operate this way considering that communication radios have been tunable in flight ever since they were invented?
Was it to save costs? If so how does it save costs considering that there must be 12 discrete, partial tuning circuits (even if it's just inductors being tuned).
Or was it maybe for a legal requirement in order to restrict glider clubs or owners from selecting frequencies they're not supposed to use?

Scratches and shakes head ...