why we don't just put one of those little $100 Spidertracks or Spot GPS things in airliners
We do. They are called Mode-S transponder...
In most cases last known position is a pretty good indication on where to start looking for the wreckage. LKP in this case is 150kms from land and a good 350km from Jakarta, closest main S/R base. With stormy weather a/c are of little use, and it will take a day for dedicated surface vessels to reach the site.