Hi,
Products are usually accessed from products listings. And so, for the same menu item leading you to the listing of the products, you would have the same menu item, and thus the same page title for each product listed when you access their product details page.
Because of this, each product has a setting "page title" that you can configure on the edit page of the product. That way, you can configure the page title for each product.
And since there is this setting in the product, it doesn't make sense to take into account the page title of the menu item, even if it's a product details page menu item leading directly to one product.