Quadruple-Digit Post Count
Funny thing is I had exactly this scenario in my post but deleted it, because I thought it would too much of stretchIf you can't get out of your chair, of if getting out of your chair is close to max effort, then how you can train your heart? And when you train your muscles, your heart is also being trained.
Look at it from the other side. If your muscles are so weak that you can't get out of the chair you're still alive, but look at what happens when your heart gets so weak that it would be the same as the other muscles being too weak to stand up, you'd be dead.
So from a medical standpoint it's low quality life, but still life, versus death. Medical professionals vow to save and preserve life. So when they have to decide between life (even low quality) and death they chose life and give according recommendations.
I totally agree with you that ideally they would recommend strengthening the heart and the body, but they follow the recommendations set by studies.
Like you said there are studies that link leg strength or grip strength (like @Antti mentioned) to mortality. There are also some that look at the correlation between lean body mass and mortality. Ultimately though these are very few, while on the other side we have probably thousands of studies regarding mortality and heart health.
Unlike you, me and the others on this forum the majority of the people don't educate themselves about strength and its benefits. That majority includes most physicians aswell. So they base their recommendations on medical textbooks and because of the thousands of studies linking heart health to mortality most of those textbooks say "train your heart to be healthy". Then the physicians go on and tell their patients exactly that.
To make it clear it's not my opinion that training the heart is more important than training the body. Just theorizing why it's the number one recommendation from most people.