I'm only suspicious if they don't simultaneously and eagerly show me their Github so that I can see what they've accomplished.
Sometimes this industry is a lot like the "finance" industry: People struggling for credibility talk about it constantly, everywhere. They flex and bloviate and look for surrogates for accomplishments wherever they can be found. Peacocking on github, writing yet another tutorial on what tokens are and how embeddings work, etc.
That obviously doesn't mean in all cases, and there are loads of stellar talents that have a strong online presence. But by itself it is close to meaningless, and my experience is that it is usually a negative indicator.
But if you mostly mean in the sense that they don't have a fancy GitHub profile with a ton of followers... I agree, that does seem to be the case.
LinkedIn on the other hand... I sincerely can't imagine networking on LinkedIn being a fraction as useful as networking... Well, at work. For anyone that has a decent enough resume LinkedIn is basically only useful if you want a gutter trash news feed and to be spammed by dubious job offers 24/7. Could be wrong, but I'm starting to think you should really only stoop to LinkedIn in moments of desperation...
I have removed my profile some time ago. While I agree with you in general, I have to say that initially LinkedIn was actually not bad and I did get some value out of it. It is a little harder now to not have it, because initial interviews scoff at its absence ( it apparently sends a negative signal to HR people ), but an established network of colleagues can overcome that.
I guess it is a little easier for people already established in their career. I still am kinda debating some sort of replacement for it, but I am honestly starting to wonder if github is just enough to stop HR from complaining.