There are tons of ways to miss companions [/spoiler]
Nonsense.
You have to go out of your way to not find the initial companions by basically not doing any sort of exploring. Shadowheart is right down the road near the giant crypt doors if she's not on the beach so you can't miss her unless you bee-line directly through the Intellect Devourers.
Same with Lae'zel. She's on the path to the grove and you can see the Tieflings at the edge of the screen as you pass near the area. Gael is only missable if you completely ignore the waypoints. Wyll you only miss if you never go to the Grove and Karlach is only missable if you never go across the broken bridge.
Again, you have to put in effort to not find these people. If you aren't exploring, that is your own fault.