Interesting, still makes me wonder why the youtubers don't suffer from constant spawns.
They don't. It depends.
I try to explain it in more detail. There are 1-3 wandering hordes spawning PER DAY! But there are several issues with them:
=> They spawn near a player and are wandering in a certain direction. And they will keep that direction. I'm not sure if the direction is random or towards the position of the player, the player had at the time they spawned. Let's assume they walk towards the player
=> If you are moving while they spawn, e.g. on a vehicle driving to whatever location, you will be far away once the horde will occur on that position you have been. You will never see them. You will only meet them in the absolutely rare case they spawned directly in a position you are moving to
=> staying for a certain time in a restricted area once they spawn, e.g. building your own base or looting a poi, they will walk by close to your location
HOWEVER:
Even it they will walk by close to your location, they are still not aggroed and therefore still walk past you. If you are in your base or looting a poi and don't aggro them, they will still just walk by and you probably won't even notice them. IF they come close enough and you make e.g. sound or aggro them with whatever action, they may come to your position. (They may also find a breadcrumb of your movement)
In conclusion: There are many spawns of wandering hordes, but from my experience, 90% of them you won't even notice, because they just walk by and you will never notice that anything happened at all. But they are still happening.