Because it's convenient, and security is a trade off with convenience. I use banking apps on my phone, and I suspect many (most, even) people here on HN (and who are technically savvy in general) do as well. That doesn't make it smart or good or correct, but I suspect that is the status quo.

I haven't traveled outside the US since before the pandemic, but these days I may only travel with a burner phone the next time I do so.

> Phones are not secure

Neither are laptops or desktops, or anything, really. Everyone needs to decide for themselves what level of security they're willing to accept, and what their threat model is.