People are different. Even if you share a lot of similar circumstances with someone, you aren’t going to be exactly the same.
In fitness though things tend to get generalised.
Being overweight is unhealthy in one camp, being over weight is fine in another for instance. Now in reality some people who are overweight will be healthy and active and happy as they are which is fine. There will be others however, for whom their weight does affect their health and wellbeing.
BMI is pointless and potentially dangerous. To be fair, I don’t know that many people in the healthy range of BMI even when they are super lean and fit so I’d tend to agree. However, I get why doctors use it, in some situations and for some people monitoring a BMI will have its uses.
A workout that is designed for a specific type of person (the busy mum, a group fitness instructor) could work for a large percentage of that group but won’t absolutely suit everyone who falls under that umbrella.
We can say if you’re in a calorie deficit you will lose wight, but of course there will be a small percentage of people who do have medical issues that mean this isn’t true.
The fact is it’s really hard to talk about things and take into account every single potential caveat. PTs know that there will be exceptions, but when for the vast majority of people or a group of people something holds true it makes sense to talk to the majority rather than the exceptions. People who are the exceptions tend to know they are and will know to follow their individual advice from their GP, PT and so on.
What isn’t helpful online is people refuting general facts because of specific anomalies. This can make people question solid advice that could help them because someone’s aunty Pat who is on some medicine that causes weight gain couldn’t lose weight even on a low calorie diet.
It’s useful to remember that not everything in the world of fitness or nutrition will apply to you. That doesn’t make it wrong or bad, just not something that would be of benefit for you to consider or incorporate. Sometimes you need to use your own knowledge or recruit the help of a PT you trust to work out what would be useful for you and what to disregard.