They just have nothing that can counter the spam.
Wargs can counter them easy, and wildmen are perfect ork-killers.
The EG from Gondor and Dwarvens is so strong how the from Rohan and Mordor to. Gondor and Dwarvens need 2 minutes to buy all buildings in the fortress. Mordor and Rohan need the same time, and all of this 4 factions can spamm units when they do this.
The point of early game with Mordor is to ignore enemy units and go for economy buildings, including the Wildmen Hut. Don't try to deny this, because I have seen you do it in a couple of our games. If a good Mordor player keeps this up, he can easily destroy all of Isengard's outer economy buildings, force him to waste money on Sentry Towers to defend his inner ones, keep him pinned in his base, then follow up with cirith ungol units and a couple battering rams to basically kill him. I've experienced this from both sides, and it is very effective. Rohan is even easier, once you have Gamling and enough peasant spam you can easily take the whole map, make an outpost close to Isengard's base and make some rams there, and finish him off.
Also, Isengard making Warg Riders? Make Farmhands/Orc Pikes for cheap, boom you win. Sure, you might surprise the enemy a bit with Wargs, but in the long run they are terrible cavalry, easily killed by the weakest pikes in the game.