I have another rather simplistic explanation: they just wanted to save money during the post-production by not having to render a cybertronian-mode for Sentinel Prime. As a bonus, they also didn't have to make the scene where he scans his earthly fire truck mode.
Why care about those details ? They don't even expect that the general public will notice that in each movie the cybertronians have a completely different reason to come to Earth. So why bother with 10sec of screen time where Sentinel Prime should be depicted as a pure-metal protoform ?
Another example of cost-savings during post-production: the scene in Africa where Megatron, Starscream & Soundwave meet.
Soundwave is depicted as arobot after the scanning of the Mercedes alternate-mode. Does it make sense ? Of course not: it means soundwave drove all the way from Chicago to Africa for the meeting
The proper way to depict Soundwave in this scene would be to show him arriving as a Cybertronian jet and have him transformed into a kind of allien humanoid shape (no wheels, no shine white painting). But no, why render another soundwave for just a few seconds of screen time ? It is better to save the money, nobody will notice