I'm guessing because it confuses them. As in- How could someone seem so jovial when they were secretly hurting so badly underneath?
Plus- it kind of vindicates them in a way. The whole: 'If only there was a sign they felt like this' (we could/ would have done more to help them.) If there were no signs, they can tell themselves they didn't miss any opportunities to give them more support.
Plus, I think people like to categorize people. When I had my IC SN welfare check, I was really busy with a project. The police officers seemed hugely reassured by that. As in- 'proper' suicidal people are maybe incapable of doing anything! Some are of course but, some aren't. There must be plenty of high functioning suicidal people out there.
I find it weird that- even people who work with suicidal people regularly seem to have a very narrow idea on how a suicidal person should behave. The people on the helpline I was coerced into phoning wanted to talk about these 'intrusive' thoughts I was having. These are presumably specialists. Why would they assume all people experience ideation in an intrusive way even?
Ultimately though, I suppose people would like to think they could 'spot' a suicidal person. So- when they can't, I suppose it troubles them. Seeing as it could well mean friends and family members could be feeling the same way and, they wouldn't necessarily know.