It's a combination of a few things

- Relative lack of homegrown talent (players with genuine affinity for the club, although this is changing of late with the likes of Wilshere, Jenkinson, Gibbs and so on)
- Lack of trophies in the last seven years
- Low upper wage limit

Which convinces players to join us when they're young (due to us paying them more) but forces them to leave after a few years (generally once they've reached their peak) for more money / trophies. Our wage structure definitely needs to be reviewed soon.