Pipe in Environment variable in fish shell doesn't execute the first command?

I want to set fish_greeting to date --rfc-2822 | cowsay -f tux | lolcat (don't judge me). When I run date --rfc-2822 | cowsay -f tux | lolcat interactively, I get the proper output (the date in a speech bubble above tux in rainbow colors). However, when I set fish_greeting date --rfc-2822 | cowsay -f tux | lolcat, I get this output:

 _ 
<   >
 - 
   
    
        .--.
       |o_o |
       |:_/ |
      //    
     (|     | )
    /'_   _/`
    ___)=(___/

date --rfc-2822

(in color, of course). Why isn't the first pipe working?