Because we can't! We've tried and practiced and strategized, but if those guests arrive before we're done cooking, we're goners. What about you?!We're pretty good at multi-tasking while we're actually cooking, but carrying on a conversation at the same time seems to require a different kind of concentration - and one at which we are not adept!
We miss steps in the recipes and forget to finish our sentences. We neglect to offer drinks and then accidentally let the pasta overcook. In essence, we have trouble playing hostess and playing the chef at the same time.
It's inevitable, though. There are always those guests who will arrive early and there are always those times when we're running late.