Why not attack the player slinging resources in team games?

I've been watching some tournaments lately (mostly Escape AoE ECL) and in team games slinging is obviously still a really strong strategy. Whether it needs a nerf or not I don't feel qualified to say but it does seem to be difficult to counter when done at the expert level, and mostly I see the team who slings, wins.

What I have rarely, if ever, seen is the opposing team trying to attack the player slinging resources, which surprises me. The slinging player often stays feudal for an extended time so would be vulnerable to castle age aggression. But I rarely see the opposing team target this player.

Is this just because the player receiving sling is applying so much pressure that the opposing team can’t afford to attack the slinger? Or are they assuming the slinging player is fully walled and can keep adding layers if needed? I am confident I’m missing something, I am just curious what it is. Thanks!