Question: When Seth teleports a plate with a piece of steak on it, why doesn't the steak merge with the plate?
Answer: Something non-biological can't merge with something biological.
Except that later on, a portion of the telepod merges with the Brundlefly.
Answer: Short answer, because the plate doesn't have DNA. Initially the pod was making synthetic versions of what was being teleported, which is why it turned the baboon inside out and made the steak taste funny but inanimate objects appeared OK. It was only later, after reprogramming, that the pods could basically identify tissue to teleport living organisms. However, with the fly present, the pod's programming couldn't distinguish there were two separate living organisms and ended up combining them. It didn't happen with the plate because the pod recognized it as a separate object.
Bishop73