I can think of a couple:

Why would anyone want the AC to resemble the front cabin of a UA international flight? Not me.

If you're seated anywhere near a galley onboard an AA flight, you're already subjected to more galley gossip than you deserve. Extend that to the Admirals Club? No thanks.

Airline employees already have access to thousands of square feet of space at the airport that's off-limits to passengers (including crew lounges). Don't think it's asking too much for employees to be barred from a few thousand square feet of passenger-only space.

Non-elite status employees would face a pretty stiff membership fee even if allowed to join. But they'd quickly recoup that in free Cokes alone, not to mention snack mix. What's an airport Coke cost these days? With access to the AC, they'd have a bottomless cup.

And employees with elite status (they exist) would save even more.

Banning employees was a good decision when it was made, and continues to have merit.