Couldn't they do that with physical SIM cards? On their end, record the IMEI of the first device they see connecting with a specific SIM card and then disallow connections if that SIM is used with a different IMEI.

I'm not sure if that's legal, but even if they did it, it's a lot more opaque. If they started doing it, many people would assume it to be a technical fault by the provider or the phone manufacturer, and the ensuing support calls and drama would probably cost way too much for this to be worth it in the first place. However, with eSIM, they get to redefine all the rules, since the customer has to learn how to use them from scratch anyway. And they also get access to nice, digital, software-driven workflows that can make the need to pay up apparent, as opposed to just randomly cutting service to the user.