Well clearly we have issues defending too so I don’t get your point. Defence as just as big an issue with this team as anywhere else. We can’t just say ‘well we’ve fixed it in previous years already so let’s move to other areas’ if it clearly isn’t fixed sadly.
We've conceded more goals than Burnley, only a couple less than Everton, one of which is very likely getting relegated - and you're wondering why we might want to improve the defence?
The issue is not the personnel - it's that the rest of the squad, in particular the midfield, isn't great and offers no protection to the defence. Varane is thought of as one of the best CBs in the world and has looked out of sorts at times here.
We only have a fixed amount we can spend each year, and pretty much every year for the past 4 windows we've spent a significant chunk on it on Bailly, Lindelof, Maguire and Varane - all of whom are still here and capable international defenders.
For me before we sign a CB we need at least 1 DM, a good technical midfielder, a RW, and a ball playing CF. After that I'd probably even have RB as a bigger priority than CB. It doesn't matter how many CBs we buy - if we don't fix the rest of the team we'll continue to both leak goals and not score enough. For me a DM like Fabinho, Casemiro, Fernandinho etc is a million times more important than another placeholder CB.