I still remember the professor in my networks class explaining how TCP worked, and then saying more or less:

Why doesn’t it send a detailed mapping of which sections of the stream have been received and which haven’t, allowing retransmission of only the dropped packets instead of what it does which is just backing up and blasting a whole new window’s worth every time a single packet is dropped? Well, I don’t know. It’d be a little more complex but the improvement in functionality would be so obviously worth it that it should. Don’t know what to tell you. Anyway, this is how it works…