The script is something like this:
Person1: Arg! This thing!
Person2: What ya working on?
Person1: is giving me trouble.
Person1 then figures it out while trying to explain it, or Person2 has a helpful idea. Happens multiple times a week. If you were working at home you wouldn't even know you were missing out on this interaction.
Also, WFH is also just too distracting if you have a family/roommates etc...
I'm sure there are plenty of jobs where regular, informal face to fact interaction didn't help me solve problems faster, but not in my line of work.
I have some interaction with Federal employees too. They have been out of the office for a long time. You now have more a problem getting a hold of someone who is nominally 'at their desk'. I don't know why this is, but its a fact.