Once again, because everyone in the first movie is a retard.
When making the film they decided that they should throw the following things out the window:
The book (and the tactics and ideas it offers)
Tactics
Strategy
Common sense
There are so many problems with the movie it isn't even funny. Besides the ones already mentioned, there is also the fact that the fleet, despite the fact that they know they can't avoid the plasma attacks from the bugs, decide to gather in one large group, basically making escape impossible.