An EV battery is not some 12V with a couple leads sticking out of it. Just like an engine, it requires rigid attachment to the frame, integration with the airflow circulation, etc. It's not just sitting in some compartment that you can open up, it generally runs the length of the entire vehicle, having a meaningful impact on structural strength. The EV pack is also significantly heavier than most car engines (~500-600kg for Teslas - you can get whole cars lighter than that). And HV connectors are a lot more sensitive than just some random wire. When it comes to engineering, designing the HV connectors to survive numerous removal / reconnection connection cycles without degradation is one of the hardest parts. It's one thing to demonstrate simply swapping it once, but ensuring reliability is a much more challenging part.
Beyond that, your comparison of a car engine not designed for swapping with an EV pack designed for swapping is facetious.