Not necessarily. There are all kinds of dogs that need to be fostered for different reasons. There's a number that have been mistreated, neglected, not trained, etc. and have issues, but a lot are just normal dogs. There's a million and one reasons pets end up homeless. It can be as simple as the owner moving or even dying. The pets they've had for years now don't have a home.

If you've observed what appears to be a scarcity of rescue dogs that don't need training or are "issue-free," that's mostly because those dogs get adopted quickly. There's no reason for them to be fostered because they can get into a home right away.

If local shelters are full of dogs that don't click with a potential foster then foster-based rescues are a good place to look, since they don't have a physical shelter they should have dogs that are ready to adopt out that need fosters in addition to any with behavioral issues. Medical rescues as well - those pets can require more work but it's on the medical level and not the behavioral level. Those are often sweet and normal pets that just need extra attention. My easiest foster was a cat with a broken leg who literally just couldn't live in a cage. His extra required care was... living in my house.

I love rescue and fostering, but it's so important to do it right. Giving a dog with behavioral issues to a foster that is unable or unwilling to address them is obviously a recipe for disaster. A ~good~ rescue should make the effort to match a foster with a dog that suits them - that's how you adopt out happy, normal dogs!

Sorry, this turned into a bit of a rant.