His movement is just very poor for a striker at the top level. He is lazy and he doesn't make runs, or doesn't make too many useful runs that either get him open in the box or have a defender tracking him and opening up space for someone else. And to make matters worse, he's definitely a confidence player - where when he's in form he'll look pretty great, but when he's out of form you can expect things like missing simple tap ins routinely or just silly unforced errors.
He's just not well rounded enough to be asked to play as a more mobile target man, he's just not got the right work ethic and/or football intelligence. He needs to be leading the line as a simple, old-school target man who's not asked or expected to move around too much. And when he's in form, he's above average at that. But he's not a top striker because of his obvious limitations and the fact that he needs to be in a more limited system to succeed.
It is hindsight... but yeah, he should have stayed at Villa. They needed his goals to stay up and their attack was focused around him and making him successful - without him they looked like total dogshit. And he never fit in with us at all... it was actually a very strange signing for a Rodgers side, as the good season we had under Rodgers involved our strikers providing constant dangerous movement. Benteke offered a much more static attack from us. And looking at how Klopp has us play and how he likes hard working strikers that move around a shitload and aren't shy to drop deep or wide to let our wingers run play off the last defender... it's no surprise he didn't have much time for Benteke.
And I think you're right about his struggles this season, compared to his form last season, where if you ask him to do too much... you'll see these kind of performances from him. But if you change the system too much to fit the more old school approach that he's proven he can succeed in... you run the risk of just getting left behind with your old tactics as modern football progresses.