An example with France, there is between 400000 and 600000 (source an article from Le Monde) illegals, meanwhile the jail system can hold less than 60000 and ar

An example with France, there is between 400000 and 600000 (source an article from Le Monde) illegals, meanwhile the jail system can hold less than 60000 and are already full. So, even if you want to put everybody in prison, you need some absolutely massive investments, and the cost every year will also be substantial (guards, food, sewage, etc, etc). And I ignore the fact these facilities were designed for adults. Update for clarity, My point here is to show that you cannot technically just "Keep them detained in the meanwhile." without doing monstrous actions at industrial scale.