I think if Transformers existed in reality, they wouldn't be biped in appearance. They only exist in our 'reality' because the toys were designed to resemble human proportions. I think an example of the odd shapes we would see in real TFs, would be characters that were not originally toys and could look a little more creative - like Deceptitran, RacknRuin, War Within Ravage, Quintessons, etc. Having toy characters show up with odd proportions shouldn't attract a question of 'why doesn't it look humanoid/biped', but all the others should attract the question, 'why do they look humanoid for an alien race'? It's like in Star Trek, every alien race was mostly humanoid (to save money on effects), and was eventually explained that there was an underlying universal DNA throughout the galaxy that caused it.
Star Wars was kinda stuck with the same problem until they had a bigger effects budget with the more recent movies and CGI series.

But for marketting purposes, the familiarity of form helps sell toys and gain viewers, so that there isn't as much confusion distracting them from being interested and involved in the story itself (and become fans/collectors).