Because subject and topic are different.
Topics, as I said, are the what you are literally talking about in the conversation. In Japanese, you don’t usually use actual pronouns when referring to people or things. That’s what the topic is for. Once you bring up a topic, you don’t have to use は and that word/phrase again until you bring up a new topic of conversation.
In other words, once you say “アリスは食べる” it’s implied that everything from then on will be about Alice, so you don’t have to say “アリスは” again. I could say, “アリスは食べる。飲む。” and the second sentence would still be about Alice. But if said “アリスは食べる。 私は飲む。” that means we’ve switched to talking about me.
As for subjects, they are what we typically think of in English. It’s the noun/phrase that does something. They are marked by the が particle. The topic is often the same as the subject. But sometimes we can’t use a subject as a topic. This is if it is new information is added to the conversation. So I can’t say, “アリスは食べる” if the person I’m talking to has no idea who Alice is and she isn’t present. I would have to say “アリスが食べる” because when you introduce a new piece of information you use が to mark the subject. From then on, you can use は to mark Alice as the topic because you both know who she is now.
I hope this helps some. It might be a little confusing but は/が are two of the most confusing particles. However, they are two of the most important ones too, so I would make sure you study them both heavily.