If you are approaching a merge with a vehicle ahead of you, the safest thing to do is keep full attention on that vehicle.
If there is a merge lane, you'll have time to change lanes after you've merged.
If there is no merge lane, it's more likely that the vehicle ahead will suddenly stop, and therefore even more important to keep full attention on it.
Drivers often try simultaneously to monitor the vehicle ahead of them, and plan their merge. As you found, it's impossible for a human to do both safely; we can't look in wildly divergent directions at once. So, a person has to prioritize in that situation and the priority has to be 1) the vehicle ahead of you, which might stop, then 2) plan and execute your merge.
If there is a merge lane, you'll have time to change lanes after you've merged.
If there is no merge lane, it's more likely that the vehicle ahead will suddenly stop, and therefore even more important to keep full attention on it.
Drivers often try simultaneously to monitor the vehicle ahead of them, and plan their merge. As you found, it's impossible for a human to do both safely; we can't look in wildly divergent directions at once. So, a person has to prioritize in that situation and the priority has to be 1) the vehicle ahead of you, which might stop, then 2) plan and execute your merge.