It's because almost all of them are indoctrinated into the guards!

5832129 Perhaps, but what if those who aren't? Are they defunct?

5832155 Perhaps they are not able to use service because they are needed elsewhere (Big Mac to help the largest export of apple products in the country, Braeburn because he's pretty much the sheriff of Appleoosa, Mr. Cake to run one of Ponyville's only commercial food stores, ect.)

5832166 Perhaps, it's a valid enough theory.

5832129

. . . and because that's how horse herds operate in real life. Mares outnumber stallions by a ratio of about 15 to 1, assuming that there's more than one stallion in the herd in the first place. The alpha mare makes most of the decisions for the herd, with the alpha stallion's duties being to sire foals, protect the herd from predators, and settle disputes between the lower-level mares.