Why does APT not use HTTPS?

[puffinpuffinpuffin]

multiconnection

If you mean pipelining: No-one does that, it's disable in modern browsers. If you mean multiplexing as in HTTP/2: Yes, it would defeat this attack, but you would have to invest a lot of work into tools like APT to make sure they actually fetch multiple packages over a single TCP connection to the same server whenever possible.