yes actually there have been a few attempts: https://en.wikipedia.org/wiki/WayUp and https://www.productreleasenotes.com/p/what-happened-to-polyw... were 2 stories i've been closer to. you might argue they are not direct linkedin competition, and yes thats the point, you have to have a new angle to disrupt linkedin, but don't doubt that all their investor pitches included a "step 3. become new linkedin"

its ofc just hard to start and grow any social network and the default is death, but i think perhaps the more interesting answers are:

1) hiring is the one area where you want longevity, expertise, trust, and linkedin just had the most time to build that up on both the platform side and in terms of the people available on it

2) linkedin actually DOES put a lot of effort into Sales Navigator and other recruiting tools that basically all other social platforms halfass. so even though you and i may not love linkedin, it is the only platform to treat salespeople and recruiters with any form of love and respect, and you should not be surprised that it does well accordingly despite it's obvious flaws.