It's designed that way in the same way as a hole was designed for a puddle*. The caterpillars are evolutionarily successful because of a "spray and pray" strategy, and other species are successful because of the easy food.
Biology is an arms race, in a sense: so everything is interlinked, and affected by everything else, even if only by distant, myriad links in an unbroken web of chains. It's the reason a lot of biologists like myself are anxious about the ecological destruction that's been unfolding for so long. Life finds a way in the long term, but short term...it sucks to be alive when many of the things you depend on aren't.
*This metaphor thanks to Douglas Adams
Because plenty of folks would have a solid down payment, or better credit score, if rent wasn't so damn high. Likewise affordable rent would make it easier for folks to move to places where they could get the type of stable job necessary for a mortgage, etc. It's not the only reason folks don't have the economic resources at hand, but it's usually the biggest expense in ones budget, no?
Greedy landlords are the problem, imho, and unfortunately every landlord except exactly one I've rented from (out of about ten in total) have been greedy assholes.
As for a fix: housing is a right, imho. I'm not an economist so anything I offer will be full of holes, but some way of securing that people have stable, safe, comfortable housing is essential. Making sure people can't exploit the need for shelter is a big component of whatever fix we need.