It's very easy to go from being a legal UK resident for years (sometimes decades), to not being a legal UK resident. You get about 2 weeks to leave, which is not enough to wrap up years of settled life when it's a surprise.

When that happens you don't have a home country that you have a meaningful connection to, and you certainly don't have a bank account there.

For some it happens due to predictable visa expiry and rules ordinary people can comprehend. But for others it is impossible to know in advance (I have some experience with lawyers who are unable to figure out how the law applies in practice in some cases - few can afford a proper judicial review to find out). There are numerous articles in the press in recent years about people who were resident in the UK for a long time, have long since settled with families, then suddenly found they lost basic rights.

The ones which shocked me most where people who were branded "persona non grata" as if terrorists, because they filed minor corrections to their tax returns - the problem with that is not only do the then immediately lose UK rights, they also get a stamp that excludes them from most other countries of the world. That's a harsh price for doing the right thing. That sounds implausible, until you know that the UK Home Office has exclusion targets, and will use any random tick-box to exclude people no matter if it's not justified and would lose on appeal in front of a judge. (Most appeals against Home Office decisions about residence overturn the decision, which shows how dodgy the decisions are, but the right to appeal has now been removed in many cases.)

Right now, there are a large number of people in the UK who do not know with certainty whether they will be allowed to maintain legal residence in the next few years or not. So far, non-UK EU citizens do not need visas or similar documentation while being resident in the UK, even for their whole lives. This means they don't tend to keep a strong paper trail. But that is probably about to change, and for many of them this already creates uncertainty about their future rights, because the UK has a harsh reputation for refusing long-term residence for bogus reasons, such as a gap in perfect record of household bills going back a few years, or failing to purchase some obscure kind of private health insurance a few years ago which nobody knew was needed but is now cited.

The situation to come is worse for, for example, non-EU spouses of UK citizens, who obtained the right to bring their spouse into the UK via EU law (the human right to a family life). Surprisingly, a UK citizen does not have an automatic right to have their spouse live with them in the UK - there is a substantial earnings threshold. This may force spouses who thought they were settled here to have to leave the UK and their families, over the next few years.

When any of these events happens, you are denied the ability to rent privately (even in cash), to have a bank account, or to use health services.

Very few of these people have a bank account in their home country. Why would they? They've been living and settled in the UK for years. Working, married, with families, all legally, until the day they find out they aren't.