With a bit of effort you can have a LinkedIn profile with a portfolio of projects. It's just a bit more typing and people are often too busy (or lazy :-)) to do it.
Also, a lot of times people want to make their profiles attractive to employers and employers want to see you working in one place for a long time (because they don't want people jumping ship in a few months).
Also, a lot of times people want to make their profiles attractive to employers and employers want to see you working in one place for a long time (because they don't want people jumping ship in a few months).