That is a crazy question to ask! Here is the answer.
Reason #1: It’s hard to articulate two syllables that end in R consequently, especially since the first one starts with T, and the next one starts with M, so we make it easier to say by dropping one of the R’s. In short, it’s just easier to say ‘tumeric’ than it is to say ‘termeric’.
Reason #2: the t-u-m-e-r combination in tumeric (without R) is already familiar; we say (tumor)(to merchants) (two mermaids). We have few combinations of syllables or words in English that have ter-mer in them, so we use the more comfortable, familiar setting tu-mer instead.