I take it you're one of these people that when you come across something you don't understand you think the way to understand it is to criticise it, rather than actually do a little research into the topic. Let me reassure you with some explanations that it's not the actors that are dumb.
In Chris's case he didn't leave to find something better. Any actor knows that leaving a show after one season is NOT the way to do that since it does not give you that much exposure and it casts doubt on your ability to stick to something for a long period of time. Instead the reason he left was widely do to disgreements with the production team. In particular the way juniors were treated on set and that the part did not develop in the way promised. In short he was not happy, and in an acting job that usually shows through on screen, so he did the right thing and left so as not to damage the show. He was also incredibly professional in that during the remaining tenure of that production team he did not do any interviews to really explain the reason for his departure, so again not damaging the reputation of the show's current regime. It was only after RTD left that the reasons began to emmerge in interviews.
In David's case there were two factors to consider, typecasting and energy. Typecasting is an eternal problem to actors playing any role long term. Tom Baker played The Doctor for 7 years and it was over a decade before he managed to get himself any significant role on TV again, and the most of his work these days relies voice parts which is a rare instance where being known for a significant part actually helps in the acting industry. For many people Peter Davison failed as the Doctor because he came straight out of the regular role of Tristan Farnon, and couldn't shake the image of The Doctor with his arm in an unpleasant place ;-)
The other factor is energy. As with Chris's case you need to love what you do, and Doctor Who is a grueling Nine Month 18 hour day shoot, so you REALLY have to love it to keep going. David did love the part, and from an emotional front wanted to stay much longer (hence his last line). But he knew that the sheer physicality of the shoot would eventually make it feel "like a job", so he decided to leave before that happened (I'm sure he was already having moments) so again it did not show on screen. I personally wouldn't want to watch the performance of an actor who's heart wasn't in it, so I applaud him for his brave (giving up a regular pay check for the sake of the show) not dumb (carrying on and delivering a sub standard performance) behaviour.
The energy thing is a valid point. Fortunately, Matt Smith seems like he has energy to spare. I'm gonna be sad to see him go when the time comes.