I could go with the obvious answer and say, "Because Hollywood is out of material" But that would not be actually true. The real reason Hollywood is quote, unquote "Ruining franchises" is because of the money they make, it's as simple as that. I work in hollywood, and I know this industry almost better then anyone.
Movies like the Transformers, and films based on Tv shows, and comics, come basically with a built in audience. Even if the movie sucks they know that fans are going to come out and support their favorits on the big screen. That's why the Simpson's movie opened with a 100 million dollar box office because that was all fans. Advertisement (even though there is a lot) had nothing to do with that movie opening as big as it did. Because come on, did the commercial convince to to see the Simpson's movie? If you're a fan you're going to see it regardless. Same with the Transformers, even though you and I didn't really enjoy the film it made a crap load of money. And it will sell a crap load of toys, and that's what it's all about merchandise. You say Michael Bay "ruined" the franchise, but let me ask you a question, how can you ruin a franchise that was based on a cartoon created by Hasbro (A toy company) with the sole purpose of marketing or advertising their toys to kids? say what you want about the film (I probably would agree with you) But it breathed new life in a stale franchise- Thus introducing the Transformers to a new generations of children.
One last thing this franchise that you're so in love with- go to the video store and pick up a dvd of the old show, there's just about as much story in the old cartoon as in the movie... and that ain't saying much.