Why are so many Premier League teams so bad in defence?

I suppose Chelsea were down to 10 men... well... 9 men for the last 10 minutes.

But I don't think I've seen so many goals from crosses in one weekend before.

I guess it is getting more and more fashionable to buy defenders who are good on the ball and can play it out, when some of them maybe have deficiencies when it comes to commanding their box and using their head.

Also I think, at youth level, a lot of wingers who aren't quite good enough to play on the wing are converted into full backs when they get to their late teenage years.  So I think what you are seeing is a lot of them are good at going forward, which is good to watch, but they're not specialist defenders, at least they haven't had as much experience at the back as their predecessors from years ago.

Kind of makes sense when you think about it, because the best players at school level rarely play in defence.  I imagine as well, central midfielders are being converted into centre backs a lot more commonly now as well.

And also, Wenger has been hopeless at organising his defence for years!

One weekend though is quite a small sample size.