Why is there no "add friend" tab on some Facebook accounts?

[Rodgers]

I was searching Facebook randomly, and found several people I knew from long ago. Most had the tab to "add friend" and I clicked it and it worked fine. They added me soon after. But a few of them just had "Follow" or "Message", but no tab to add friend/ request. How do I send them friend requests?

[Madihah]

They're probably pages you can only follow. They have that for some celebrity accounts. You can only be friends with them if you have an account like that. You should message them asking why their accounts like that.

[Cassie]

There is a setting for "Who can send you friend requests" in the privacy settings.
Those people probably restricted it.