That is the same guilty rate as Israel’s internal convictions and is caused by police not bothering with anything that is not an easy win.
The administrative detention also has judicial checks and limitations. Also, these are not random people picked off the streets, these are terrorists or would be terrorists where, for the most part, the evidence for detaining them is too sensitive from intelligence point of view to share in a trial.
I think it is quite obvious: to destroy the Hamas infrastructure and to kill as many Hamas militants as possible, while minimising Israeli casualties.
If you have a better suggestion for how to destroy Hamas’ ability to continue to attack Israel I’m sure they would love to hear it.