"Gave" is the key word. You got TCP for free. There were other competing network protocols. You might ask, why didn't those get used instead?

You are right, but I was referring to the fact that their ideas are not necessarily the best ones. Edit: because they were too optimistic, they left the security problems behind. Same with this kind of problem.

Authenticated TCP wouldn’t have been feasible on early networks due to hardware limitations. But here in the present, you could certainly build it. As long as it works on top of IP, nothing is stopping you.