I guess because you can hold a single hamburger as a serving, but not "a lasagna." You can have SOME lasagna, or a serving of lasagna, but "a lasagna" means an entire pan of it. If you're having one serving, you'd say "I'm having a hamburger" but "I'm having lasagna."
If someone one asks what you are serving for dinner, you say "I'm making hamburgers" (no article) but you can say "I'm ma