The UK, Ireland, and just a handful of other countries, do not have border control checking the passports of departing pax as there is no point. It is a waste of everyone's time but most countries do it.
So the police put out an all points to border posts to search for Mr/Mrs XYZ. The check in person is not security. They check the passport at check-in, true. That could alert a flagged person, but does it? Is their system linked in? If not then a APB to airports in Schengen is useless. Further, if police wanted to track a suspect's moments across Schengen they could not. XYZ checks in for flight ABC, but in fact swaps boarding passes with an accomplice and travels to DEF and disappears. The police waiting at HIJ to intercept are left dumfounded. It does seem strange in a era when security seems to getting tighter.
I'm not a great supporter of Big Brother, but it seems uncoordinated when compared to some of the petty trivial security checks that are enforced.
Previously I have seen pax removed form the departure passport check (Schengen) as they had been flagged as not having paid certain fines, or alimony or tax etc. That is why departure checks were thought useful. However, I understand that it violates the principle of Schengen and as checks can not be enforced across all forms of travel, why bother with any?