Er, because they are???
Laporte has been outstanding all season (Spurs CL leg2 aside). Silva is well, probably they best player ever to pull on a light blue shirt. And Sane is one of the brightest talents in world football, a game changer, a genius.
Zinchenko is a very limited footballer. His footballing brain runs at 0.5 MHz, compared to Silva's 4GHz 10-core. He receives the ball, ponders about for ages, and then passes it backwards 90% of the time. He has little natural ability and is pretty ropey at both attach and defence. But he tries hard. I thought "trying hard" was not the benchmark we aspire to.
Seriously, I don't have a downer on the lad - he seems like a good bloke and he does try. I just don't think he's that good tbh, and for me is an obvious weak point in our team. Left and right backs have been a weak point of ours ever since Zabba went past his prime. Left back we've been weak for donkeys years.