There's a whole army of Decepticons taking over the city under Sentinel Prime's command and the Autobots seem able to offer much resistance. Might as well sit back and enjoy the show.
Btw, the comic adaptation provides an explanation as to why the Ark happened to crash on Earth's moon of all places in the universe...
"Sentinel was to seek out the last Star Harvester - to fuel our empire's expansion - then rendezvous with me. But fate waylaid us both, before drawing us all together - to Earth." - Megatron