It could be that the autobots simply need the raw power of dino sized/shaped robots so the build the dinobots.
Then why not just build huge robots with guns and rockets?
This just doesnt make any sense. For them to scan actual dinosaurs, that would mean that they would have to come to earth before even the Primes came.... Ubsurd. Or that they scanned them in a museum or an amusement park. But even if the Dinobots are ******ed, wouldnt any other bot point out that they did a bad job at camouflaging themselves? So sooner or later they would be forced to scan something else. Then, its the matter of mobility. The others would just drive around. How would the dinobots go around?
It just wouldnt work. Especially if they are like the Beast Wars robots with actual organic skin (alice used a hologram right?), because lets face it, the point of the franchise is about robots turning into cars and jets, not terminators.
The only way it could work is if 3-5 new autobots (not the dinobots) came to earth and were assigned to a spying mission. Lets say to spy on a goverment building next to a museum. Or maybe even protect it. Maybe they could be protecting Sam's university. But the university consists of a thousand buildings, doesnt it?
So anyway, the museum happens to host a science fiction expo, so they try to hide there, scanning and replacing the robotic dinausaurs (that happen to look like the G1 dinobots) that were on display. And they stay there spying or guarding until the decepticons attack said buidling and they use their dino modes to "bring the rain" on the decepticons. They later abandon those forms because they are impractical for transport or camouflage but they comment on how awesome and powerful those beasts would have been at that time.
Now, in the beginning of this i said that i wouldnt use the actual dinobots because after the nod to them, the robots would get more conventional alt modes and it would look silly seeing Grimlock transform into a car for the rest of the movie. Besides, you really want to trust the most stupid bots (and i dont mean that as an insult to them, but rather that they werent really bright, were they?) of the franchise to Bay? You know that they will be in more sex jokes than fights right?
That would be a nice nod to the dinobots without bogging down the script. That way the fans get to see some dinobots and the genaral audience wont be scatching their heads on why those robots would even bother transforming into dinosaurs.