You have another good point. Maybe I don't need to emulsify and just mix well, but WC patties are processed to some extent. Might be nice not to have to clean up the food processor... OK. Breakthrough... Just watched a few videos of WC preparing the burgers and they season the burger. Well duh. If you add salt to hamburger it hardens... Also, the package of frozen burgers clearly states 100% beef, nothing else.
I think you hit the nail on the head! I was about to reply saying that if you add salt early and you work it in the meat what you get is a burger that turns out "dense" or as you are describing, "hard".
Do a google search on "The Burger Lab: Salting Ground Beef" and they will show you what I think you are running into.
Basically when you add salt to ground beef the salt helps make the proteins bind together to produce a thicker/denser/harder patty. If you mix the salt in and really work the mixing you overwork the meat protein to break down which then wants to bind together via the salt to be even more dense!
Here is what they/I'm talking about. Patty on the left side is perfect. Patty on the right side has had salt mixed in and the meat worked quite a bit while mixing which results in a dense/hard hockey puck patty!
I have run into this issue and I employ a simple solution.
In a large bowl I mix the meat and all seasoning EXCEPT SALT into the ground meat very well. My seasoning is Pepper, Onion, and Garlic.
I take the bowl out to the grill (my grill has side panels) and when the grill is hot and ready, I mix the Salt into the ground meat and immediately form a patty and throw it on the grill. Form another patty and throw it on the grill... repeat until all patties are on the grill.
This process gives me the wonderful mixed in seasonings and flavor WHILE keeping the salt from having enough time to do it's thing where it binds the proteins to make the patty come out dense/thick/hard!
Another solution is to premix all seasoning EXCEPT SALT and then make patties.
Before you go and grill the patties you season the outside of the patties with salt (flip to salt both sides). Then you don't run the risk of the salt binding the meat inside the patty. This is the faster easy way but doesn't taste as good to me.
Anyhow, give this a shot and let us know if it fixes your issues and helps with the conclusion about salt we both have come to. I can tell you I don't run into the issue anymore and my burgers have maximum flavor! :)