In case it needs to be spelled out, the problem is Europe has a maritime duty to rescue [1]. Once rescued, it has a duty to consider asylum and take care of the migrants/refugees [2]. Essentially, once SAR identifies a migrant vessel in distress, the closest country is forced to admit them.
The duty to rescue means "a state cannot legally prohibit its vessels from rescuing persons at sea." This technically covers search. But some Mediterranean governments have taken the view that if they don't see the migrant vessel sink they don't have a duty to rescue.
Rafts leaving Libya are incapable of reaching Italy on their own and are scuttled when they see a ship that could rescue them.
The expected value of this gamble is higher when search efforts are high. Migrants would not take this gamble if they were sure it would not work, right?
The duty to rescue means "a state cannot legally prohibit its vessels from rescuing persons at sea." This technically covers search. But some Mediterranean governments have taken the view that if they don't see the migrant vessel sink they don't have a duty to rescue.
[1] https://blogs.law.ox.ac.uk/research-subject-groups/centre-cr...
[2] https://en.wikipedia.org/wiki/Convention_Relating_to_the_Sta...