Because it's not (a deterrent.)Listening to an 'expert' on the telly just now, he says we have 3 days to identify someone we are planning to send back, then we have 14 days to notify the French. It can then take up to 3 months for them to be swapped and that’s if there are no legal challenges. That doesn’t seem lime much of a deterrent to me.
It's tinkering around the proverbial edges of a significant problem.
For some reason I keep thinking of this whenever the scheme is mentioned:

