Roaming has a bad reputation, and it's earned. But it's worth understanding what's actually happening, because the answer isn't "data is more expensive abroad" - the towers, the spectrum, the equipment cost the same wherever you are. What's expensive is the arrangement your home carrier made on your behalf, without asking.
None of this is a scam exactly. It's closer to a toll booth on a road that used to be free - added after the fact, and easy to avoid once you know it's there.
What roaming actually is
When you land somewhere your own carrier has no towers, your phone finds a local network instead and asks to borrow its coverage. That local network doesn't know you, so it checks with your home carrier, which has a wholesale agreement to let its customers use foreign networks and settle the bill later. This handshake happens in seconds and is genuinely clever engineering.
You never see any of this. Your phone just shows bars and a different network name in the corner, and everything otherwise behaves the way it does at home - which is exactly why the pricing behind it comes as such a surprise to people who've never had reason to look.
The problem was never the engineering. It's the settlement. Your home carrier pays the foreign network a wholesale rate for your usage, then charges you a retail rate on top - and outside reciprocal zones like the EU, that markup has historically been steep, because there was no real competition forcing it down.
Why it's priced like a minibar
A minibar can charge £6 for a Twix because you're not going to leave the room to find a cheaper one. Roaming worked the same way for years: once you'd landed, switching carriers mid-trip was impractical, so your home carrier could charge close to whatever it liked for the privilege of staying connected. Regulation has capped this inside the EU. Outside it, the minibar pricing largely remains.