The shortest path between two points is a straight line but annoyingly, when lifting the bar from the ground, it makes a great deal of sense to start with bent knees. Sure, you could stiff legged deadlift the barbell up but then you’d be hard pressed to jump it up to the catch position (it’s nigh impossible to jump when all your joints are locked out).
So the dilemma is this: how do we pick up the bar with bent knees but not arc around our knees but instead bring the bar towards the hips in a (mostly) straight line? There are probably lots of answers to this but the simplest one is to fix your set up. If the bar is too close to your shin (like a deadlift set up) and your weight is in your heels (again, like a deadlift set up) at the start of a clean, then things are liable to get ugly. Instead, start with the bar over the toes (usually the first lace on your shoe) and your weight in your midfoot.
Good setup yields a good lift. Watch this video for more setup details.