It's not really anything to do with eSIM. If you used a physical SIM from the same carrier with the same settings, you'd have the same experience.

When connecting via a cellular data network, all data will be routed via your carrier's Access Point Name (https://en.wikipedia.org/wiki/Access_Point_Name).

Most carriers only have servers in their home region capable of handling the traffic. It is possible to have regional servers for this, but most carriers don't bother with the expense. Roaming data is usually expensive or restricted in usage - so there's traditionally been very little demand for higher speed connectivity.

Travel eSIMs are usually just a regular SIM with a very limited plan from a carrier that has favourable roaming deals.