Are the US eSIM only iPhone models rated better than the EU traditional SIM models? I was under the impression that they're all pretty water resistant these days.

Rating is just a rating.

I'm sure a phone with less external openings like that would still on average fair better against water.

The main benefit is that it's easier to have multiple eSIMs than to have multiple physical SIMs. Sure you can only have one active simultaneously (I think?), but that's better than the status quo of physical SIMs.

I'm a US/UK resident, and I use an eSIM for my US provider and a physical SIM for my UK provider. Since I use both numbers for various localized services like 2FA, food delivery, etc., the ability to have multiple SIMs has been a huge improvement compared to just a few years ago, when I had to have two phones to be able to log into my bank.

The iPhone can have 2 active simultaneously and quite a few stored.