Kneeling is 4 TUs. Standing up is 8 TUs. For these 12 TUs, I'm likely to find a much better cover instead of just hugging the ground ineffectively. And while the accuracy bonus is nice, going 3 tiles towards the enemy will improve this too.
Sure, there are cases where kneeling is beneficial, mostly when sniping or when behind a low cover. But using it as a default manoeuvre each turn is more trouble than gain.