Because gelatinous blobs are typically hostile and sinister alien life forms in the interstellar community, I guess.
It's similar to their deal with snowflakes, which the Solomons mistook for albino brain chiggers.
Having just watched the show for the first time, with early episodes - such as where Dick states "If you see a blob of purple with a watch, don't take it" - their native form is not dissimilar to jello, and if it doesn't move they think it's dead.
shareI always put it down as an homage to "The Blob" Perhaps somewhere they had run into the "Blob" and never forgot the mind-numbing fear.
Or perhaps a Non-newtonian fluid like Jello just freaks them out.
Or maybe they think Bill Cosby is nearby?