Would you send an email without a subject line? Why should Teams be any different?
Go vote for this UserVoice request to make the subject line appear by default when starting a new conversation in Teams. This helps keep individual conversations on topic, helps users remember to use the threading features, and makes searching for older conversations easier!
Honestly, I think it is because internally at Microsoft I've seen examples where they are much better about creating distinct Channels for topics so all chat inside that Channel is relevant and timely based on activity. I'll bet subject lines on the average only get used when someone is planning on tagging in people or other entire channels that don't usually frequent the originating channel. Or the entire team, as more of an announcement type action.
Even in a distinct channel, every message has its topic. and browsing through a channel and scanning for a particular message is that much easier, you eye can just jump from bold subject line to bold subject line.