A lot of “traditional” industries still run on TUIs/even as400 systems, such as freight/trucking companies, banking, medical, travel (saber, etc). And many companies in those spaces who appear to have migrated away from those systems are likely just using a modern gui interface that still uses those old systems behind the scenes.

It’s generally because 1) they’re actually reliable 2) they contain custom business logic and rules that are not documented well/at all and hard to replicate 3) the people who set up all of these things have retired and no one is left who understands any of it