Welcome to what happens when you put digital projectors into theaters. Since they can run advertisements any way they want, without anybody having to cue up spools of film or whatever, then that's exactly what they do.
I agree it's annoying, but all the local theaters here do it. I'm resigned to it.
I don't think that's the cause at all. If anything, I see far fewer ads after the start time than I used to. Back in the early 2000's I used to see the same thing that Glenn described at my local theaters. But I think most chains have heard this complaint and are now resigned to putting the ads before the start time. It's been ages since I've seen anything but trailers after the advertised start time of a movie.
I would say that the theater owners realize that there's more competition both at home and from other theaters. When I was growing up, there was a single movie theater in town, and I grew up in a highly populated area. I live a little ways from there now, but in 15 minutes I can get to 7 different movie theaters from five different chains (and the two Regal theaters are very different styles). I know that if one of those theaters showed ads after the start time, I'd never go back to that one.
There's an AMC a little farther away that has its own produced segment that runs before the movie called "The 20," where they show promotions for products and TV shows (usually all terrible), and just before the advertised start time they wrap up all the stuff you saw, or didn't see because you got there at the end of it. I'm fine with that. They can do whatever they want before the show is supposed to start.
(Alamo Drafthouse has a theater way, way, way out west of the city of Houston, but nothing in town. Yet.)
I'm very sorry. I feel so lucky to have one a mere 10 minute drive from my house. I adore the Alamo.