That's part of it, but intestinal parasites also interfere with normal digestive processes. Their presence and secretions (waste products and the compounds they use to prevent being digested) can cause a lot of problems. It's common for people with intestinal parasites to have reduced appetite, intestinal inflammation, and digestion issues.
The weight loss isn't just a result of the parasite competing for nutrients, though that doesn't help.
I thought of a refinement to my previous: tapeworms attach themselves to the host intestinal wall only to secure themselves; however, they feed directly from the food in the digestive tract; they do not "suck nutrients" from the host's bloodstream.
I'm no expert on invertebrate metabolism, but it does not need to be more expensive than host metabolism, it's simply a redivision of the pizza-pie with more slices going to the growing worm, and fewer slices for the host.
consider the possibility of there being n worms in the host (person), where n is greater than 1, maybe even significantly greater than 1.
so if one worm takes m slices of the pie, and there are n worms in the person, there will be m x n less slices available for the person, which, depending on the values of m and n, could be quite high, and therefore much more detrimental to the person
