That makes sense, but it would still be a huge improvement to eliminate gaps everywhere but the floor. The cost argument makes sense, but it doesn’t explain why it’s ubiquitous in the US but not in Europe, since the cost-saving incentive should be roughly the same in both regions.

Perhaps it started in the US because of safety or puritanism, but has now simply become a de facto standard in the US building industry. I doubt most building developers still think about safety or puritanism and specifically request this type of bathroom walls.