Because you turn into a ram OR wolf. Not both.
Fighters can swap weapons each attack, use polearm master for an extra attack, then use action surge for even more attacks.
A level 5 moon druid could push, push
A Eldritch knight 5 fighter could prone, push, push, prone, cleave.
Eldritch knight gets 1/3 spell slots
Moon druid gets 1/3 masteries (effectively).
I am not suggesting anything overpowered.
And if it somehow is, it could be limited to once per turn or some such.
Right. But I think you're missing the fundamental problem. The actual crux of the disagreement.
Weapon Mastery is a new feature being added
specifically to differentiate the martial classes. They are trying out this feature so that the warrior classes (Fighter, Barbarian, and Monk ... although the Monk utilization will be revealed whenever they get around to that).
What does the mean? It means that
other classes do not get it.
Rangers? Nope. Paladins? Nope. Rogues? Nope. And spellcasters, including Moon druids and valor bards and war clerics? Nope.
This is not a bug- it's a feature. Everyone who loves one of their classes can imagine why they want this feature! If you're a Rogue fan, you are imagining your rogue all awesome with a dagger, or whatever rogues are using now. Paladin fans can't imagine Sir Smites-A-Lot without being the bestest sword-y guy ever. Strider McAragon is dreaming of his scimitars, or whatever.
And, of course, every gish or spellcaster wants it, because of course they do. Heck, you think that the Moon Druid wants this ... what about a HEXBLADE???
But that's the point- this is a feature for these classes. Just for them. It's not like people have all been, "You know who has been too powerful for too long in D&D? The martials! The three classes known for not casting spells." So if you want to take advantage of this, then you have to do it the old fashioned way- multiclass or look for a feat (because of course there will be one).
As for the specific case of the moon druid, it's my understanding that it got nerfed a little in the last playtest. I'd concentrate on asking for specific features for the moon druid, instead of cribbing features from the warriors. You'll get a lot more support (IMO, YMMV, etc.).