In the US at least, health is conflated with goodness so saying that being fat is unhealthy could be misconstrued as such

Edit: Also it causing health issues doesn’t even mean it’s inherently unhealthy, just that it’s likely to cause health issues with a lot of people who experience it. But what actually counts as unhealthily fat presumably varies from person to person so just saying it’s unhealthy by itself could be uninformative and outright harmful