We don't know why. We just hope it is all the WRs fault but it could be a combination of both ie WRs and an inconsistent QB.
It is combinations. Receivers and the QB when he doesn't set his feet. When he sets his feet he has thrown into tight windows. Thrown touch passes. And thrown with speed and velocity at times. Having your guards and center get driven back into your face means that sometimes the most accurate pass is only "catchable" and he can't put it on the numbers. And of course people are human sometimes he might sail one that has nothing to do with anyone but himself.
So if you are gonna have expectations and judge him then you need to grasp these things. And be able to see that what he lacks is easily fixed and can come in time & proper coaching. There is nothing about Mitch that seems unfixable or broken. And plenty of things that I see that tells me he can easily be in the upper quadrant of the league as a passer. Like any QB time to settle in and have things that he is processing now become second nature & instinct later.
All this play time is giving him valuable experience. And even if the coaches are stunting his growth at least their will "be" growth. This off season and better talent around him will make a world of difference. But if he has to learn a new offense then that will stunt his growth at first.
He may never be Tom Brady or Aaron Rodgers but there is no reason he can't be franchise material. There are plenty of great QBs he could become as good as even if he is never the best. The most important thing is to have a great "team" so it will take more than him. And the coaching has to be better if you wanna win a SB.