Pilot shortage IN EUROPE like you say has nothing to do, because it is a complete myth-
Agencies flourish simply because there is too much low time pilots in europe for very Little positions. Not so much highly experienced pilots but still enough, its just they are not avid looking to change Jobs.
Experienced FOS are a proven commodity who companies see as an overpriced asset, and captains ( which usually implies experience ) dont come cheap. Hence looking for this things come as a costly search for an airline, so they outsource this " search". They want to weed out the 200 hr pilots which are dime a dozen but they still need experienced guys that are ready for upgrade, this takes a lot of time looking at cvs, phones, emails, etc.
Companies therefore will hire this agencies to do the search and for every guy they find they will get a comisión.
Result are those who you are complianing about flashmobbing everyone into linkedin in search of their comisión, they dont care bout you or the airline, they only care about getting their comisión.
Nothing more, nothing else. As simple as that, dont read more to it. This is all a result of too much pilots in Europe ( like you said ) and companies dont wanting to pay, that is the bottom line.