[joshka]

There's another article on the site [1] that explains this in detail. It seems that you can drop the "watashi wa" only when the context of the statement or answer is obvious from previous parts of the conversation.

[1]: https://8020japanese.com/wa-vs-ga/#comparison

[iamnotlarry]

As others have commented here, Japanese relies heavily on inference. Subjects and topics are very commonly dropped.

I shouldn't say that the language relies on inference. Rather, speakers of Japanese rely on inference. It's like chaining functions in some languages.

Car.rent().driveTo(work).park().driveTo(lunch).wash().

By the time you get to the washing in the conversation, the car is hard to find in the sentence, and the driver never does appear. But they are both there if you go looking.