Because they're delicious, but to be fair to a great bird I'd rather give them a sporting chance.
If a lurcher is catching them then there's a good chance they're just after coming in from their migration and are weak. The full moon at the moment means they've more or less just landed.
I shoot them (when I'm lucky enough) and if any of you lads are lucky enough to get one in the bag thanks to your lurcher do yourself a favour and have him for your dinner, just give the head to your ferret.
Strangely I always leave the head on, as traditions favours, when serving them roast for a number