It's because we (the government) have decided that they shouldn't have to pay if it's "hard".
I barrowed money and went back to school later in life when I already had a family. Long story short I ended up divorced and a single custodial parent who's ex-wife never paid a Nickle.
I could tell you all about being broke and living hand to mouth (for years) but I'll just tell you that they came after me for that student loan money with a vengeance. I actually did get some temporary relief from making payments but they charged me interest the whole time so it just sucked me in deeper.
In time, as I got more settled in my new career, worked my way up some and my income increased I got it all paid and everything worked out. I paid every cent and a bunch more in crazy high interest. There wasn't any of these loan forgiveness deals they have now.