Why does PIE *weydtos give PGmc wīsaz not wīssaz?
compare Pgmc *stassiz, *gawissiz, *kwissiz
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It probably did, but one of the last phonological changes that happened to create what we call ‘Proto-Germanic’ was that long *ss was reduced to short *s after long vowels or diphthongs. More controversially (but no less correctly if you ask me), this didn’t apply to just *s(s), but to any geminate, resulting in the loss of overlong syllables in Proto-Germanic. This latter, less canonical, viewpoint is part of Kluge’s Law.
Examples with *ss include, giving a (Post-)PIE form a pre-PG form, and the ‘classical’ PG form:
Examples with other consonants include:
There aren’t many exactly parallel cases, but there are some. As you can read about briefly in the Wikipedia article linked to above - and in more detail in Guus Kroonen’s PhD dissertation – the issue has been mired in controversy for over a century.
Not, however, when it comes to *ss becoming *s after long vowels and diphthongs – that bit is pretty universally accepted.