That's a long time to be on children's tv. Most people would have moved on long before then.
I'm guessing this is either because they have chosen to stay in children's tv, or have struggled to find other presenting roles.
I don't think it would work if they presented "adult's" TV, it wouldn't be the same. There seem to be quite a children's TV presenters that have been doing it for a long time: Sam & Mark (11 years), Barney Harwood (14 years), Justin Fletcher (19 years), Naomi Wilkinson (17 years) and Ed Petrie (12 years) just to name a few.
Sam & Mark are not unique to CBBC, their first public "appearance" was back on Pop Idol for a start, and they also do a radio show too. Sam Nixon, it is stated, appeared on an episode of Fun House, while Justin Fletcher technically started out on GMTV.
Let us not forget Michael Underwood as well. As well as his appearance on Crystal Maze in the first Christmas special, he won his CBBC presenting job on Gaby Roslin's Whatever You Want show, stayed there for three years or so and then moved to ITV, but he has since moved onto more mainstream presenting roles, though his wife retains links with children's programming.
Up until his death, Mark Speight was 14 years and counting.
Of course it's far easier to build a career on the back of being a Children's TV presenter at the BBC rather than at ITV - with the exception of Stephen Mulhern, most if not all of the other regular presenters that started on CITV appear to have disappeared back into obscurity.