Take a look at the G1 figures and how "well" they translated from cartoon to real life. Now imagine those figures on a larger scale beside actual modern day cars and military vehicles. Would they not seem out of place? In terms of the live action movie, The Transformers are an advanced alien race who can adapt and blend into to what we, as in present day earth, have now. Why would they choose to look like 80s cartoons when they landed here
They looked that way in the 80s because that's the limit of what animators could do at the time. Which is why each new series, for the most part, is more detailed and animated than the last.