I'm assuming some of the reason is because they are under contract to follow the plot of the movie. Are they having to only include movie characters, and not add anything additional? This would obviously have some awesome potential for an open map type of game. Lots of possibilities. If they ever finish making the movie series, do you think they would be allowed to make an unrestricted game out of it?
I fucking loved Chromehounds, which was pretty much a transformers game, you built mech's then fought in war's for one of three factions. It was pretty much and you had to be in a clan which made it awesome. It was almost an MMO with Mechs First game on XBOX I really wasted a shit ton of time on
Try the demo for War for Cybertron. It has no movie tie-ins, and is surprisingly fun. Plus it should only be around 20 bucks now.
The reason they suck is because they spend barely any time developing them. There just isn't enough time to make a great game in the time frame they are given. It's why most movie video games suck.
It's like all games based on movies suck. Common knowledge. They don't give a shit about the quality of the game they just want a profit from merchandise.