Wrong answers only
fr/ i think it's because dummy and dumb are two different words, even though now we use them in the same context.
dummy meaning a replica of a person, meaning someone who isn't original in design or thought,
dumb meaning you can't talk, which has shifted to meaning someone who is of low intelligence.
The actual answer is that the "b" in "dumby" is difficult to pronounce clearly and so people would end up slurring over it, resulting in a pronunciation that sounds like "dummy".
The same reason that we call a corpse that has been embalmed and wrapped in linen a mummy and not a mumby.
Fact: The reason "dumb" has a b in it is that people actually used to say that 'b' but the English language experienced a sound change, where any b that occurs at the end of a word and after an m was deleted (silenced)
Compare for example, "lamb" "tomb"
Fact: But that's only when it's at the end of a word. If it were, say, in the middle of a word, and had a vowel after it, the 'b' can re-appear, as in 'dumb > "Dumbo" or "thumb" > "thimble"
hypothesis/speculation: Thus, 'dumby" might be pronounced something like "dum bee". But since we say "dum ee" and not 'dum bee", people chose to spell it without the 'b' to make the pronunciation more clear.
(sorry to preface my sentences like HK-47, but I wanted to clearly differentiate the facts from what is just my speculation)
