Why don't they make the Simpsons characters grow up?

You're asking why the characters don't age? It's a cartoon and the production team wants to play around with the characters the way the are using as many ideas as they can come up with and keep them that way. The characters are stuck in what's called "the floating timeline".

The production team behind THE SIMPSONS probably thinks it's easier to keep characters the same age year after year because it probably makes them easier to work with.

I think if characters become popular then people want to keep them around for a long time and they probably think it wouldn't work if they aged naturally.

There was the comic strip FOR BETTER OR FOR WORSE. In the strip the characters actually progressively aged. However at one point during the strip's run the strip's creator decided to reboot the series, making the main characters the age they were when the strip began again. The creator probably thought people got bored with the characters in the original strip, so she probably wanted to keep things interesting by rebooting the series.

Hope this helps.

maybe they wanted to keep it that way

Let's see . . . Bart was 10 in 1988, so he'd be 43 now.  Lisa would be 41.  Maggie would be 34.  Homer would be close to 80, probably long dead by now.  WTF kind of Simpsons would that be?

The whole advantage of animated shows is that the kids don't grow!  Look what happened on the Cosby Show after a few years.  They had to keep bringing in new kids as the old ones grew.