Almost every shampoo manufacturer on this shelf produces bottles with a flat top and bottom.
The one major exception is Dove. Their men's and women's shampoo both have oddly angled tops that make it impossible to store upside down.
This 'anti-customer' design is very much the opposite of 'human-centred design'. It favours the product's company, deliberately making it difficult to use the last bit of their product, and giving the illusion of a larger bottle - both increasing sales and reducing consumer value.
You will also notice the incentive in stores is to create multiple look alike products, rather than a single well designed product. As the single product would get lost on the shelf, where the advantage with multiple products is it both (a) takes up more shelf space and (b) makes item-to-item comparison shopping more difficult.