Hello tambor
I like your enquiry, trying to understand. And also your attitude as in:
So this, put gently (I hope) is my attempt at explaining how I think it works. I sense a degree of resentment amongst Swiss people, towards that certain sub-set of the English-speaking immigrants which assumes that just because English is an internationally used language (and the language of the technical or trading sectors, etc.) the Swiss locals are obliged to speak English to them, too.
This has levels, of course, since many of the people living here are not Swiss, but have also immigrated. They can function, in everyday contexts, mainly because they have learnt German (or French, or Italian, depending on the region). My friends and neighbours from Turkey, Sri Lanka and the former Yugoslavia can communicate with one another, with the Swiss landlady and with the Swiss government clerks, because they speak German. The Chinese doctor and the US American dental hygienist deal with their patients in German.
Anyone who does not learn the local language stays in the expat-English-speaking-only-bubble, some even for years and years, and thereby remains forever an outsider from Swiss life. (To be sure, there are many who do just that, and some of those are IT specialists, so perhaps one of them will reply to tell you how he/she found a post.)
Some employers would rather not risk offending their Swiss employees by requiring of them to speak English to the IT guy (or anyone else). Many large companies are moving away from the formal form of address ("Sie") to the informal ("Du"), so as to promote interaction and facilitate conversations amongst people of different ranks and departments, e.g. in the lunch-queue in the canteen.
Finally, it could be (though this is conjecture on my part) that the sense of separateness you describe, between the IT staff working away in a back-office with no contact to the clients or the finance department, may be less marked in Switzerland than in your home country. Several scenes come to mind where the developer or analyst was called right into meetings with the senior decision-makers, and it was not an absolute requirement, but definitely much appreciated if everyone present (no matter their mother tongue) could participate in German.
Therefore, I think Adrianlondon's advice is very good, to state on your applications that you are willing to learn German, as long as, in fact, that is the truth... which I'd hope. And get started learning right away, so that if you get an interview, you can already greet any potential employer... who will then accommodate you by switching to English.