This is a difficult question to answer, since there's no technical reason why this is true, so you have to speculate why the developers haven't chosen to address the problem.
There's a couple of possibilities, but mainly it's one of compatibility. SSH is 20 years old now, and has had a stable format for key storage for a long time. If you suddenly changed that and made brute force attacks far more difficult, you'd break compatibility. Everyone hates breaking backwards compatibility, so you'd better have a good reason to do it when you do.
Also, consider that the ssh key format is around 20 years old now. That's 20 years of Moore's law. In 1995 a high end computer was a pentium 133. In 2015 we have multi core processors running at 1.8 ghz.
If you look at code cracking speeds of 1995 vs 2015 for RC5, you'll see 246,000 crypts/sec for a Pentium 133 in 1995, vs 6.1 billion for a modern Athlon multi-core processor. That's 25,000 times faster. So you can see why when the standards were created 20 years ago brute forcing passwords wasn't as much of a concern. Obviously it is now, but backwards compatibility makes the switch not as easy to simply create it.
The other reason is that people consider private keys to be something that attackers don't have access too, and can be kept "safe". If discovered, they can be replaced. That's probably more of a bias towards believing that keys can be kept safe, and discovered if stolen, despite many break-ins being discovered long after they happened.
Ultimately there's no good technical reason why a better key derivation algorithm couldn't be adopted, it's more likely that nobody has made a stink about it in the SSH world. That's normally how long lived standards get changed.
TL;DR Increased brute force speed in the past 20 years has made passwords require more and more entropy. Using better techniques to generate passwords from can mitigate the vastly faster CPUs we now have, but the need for backwards compatibility has delayed moving towards better password deviation techniques.