eSIM is essentially a client-server protocol for provisioning secrets into an embedded SIM (whether discrete chip soldered on the mainboard or emulated by the modem).

The QR code you get when you purchase an eSIM is merely an access token to initiate the provisioning process. Some carriers may make these single-use, or attach extra restrictions such as fees if you want to get a new one, or restrictions they themselves don't know about like that you must be on an IP from your carrier's home country to reach the provisioning server (good luck debugging that if you're not already aware of it - and no, on-device VPNs won't save you as the OS will not use your VPN for this traffic).

Even the mechanism that allows you to move an eSIM from one iPhone to another requires carrier involvement, which they have to support (internally I don't believe it moves anything, instead merely requesting a new SM-DP code in the background and sending that to the new phone). It doesn't work for all carriers.

Oh and you already need to have some existing IP connection to provision the eSIM in the first place, so first-time provisioning is tricky. I'm sure there is a workaround for it, but again carrier support varies.

TLDR: it allows the carrier to interfere when provisioning or moving the eSIM which carriers can and do take advantage to make the process more costly/painful and discourage easily using alternative carriers.