i ran a music store years ago. there was a Vox modeler that was very popular.
people constantly brought it back damaged because the built-in expression pedal physically broke. mostly because they picked it up by the pedal, also just because it dropped or broke from heavy usage.
we didn't have parts to fix it, and all these people just had a board with silly looking broken pedal area that was useless. other brands too, but mostly this Vox unit.
some people want a larger pedal, or smaller, or on the left, or far to the right so the mic stand leg is between the switches and pedal.
i think it's a good thing they aren't included personally. it would increase cost and size of the boards, and many of the reasons mentioned above would apply.
fortunately you can pick up your favorite expression pedal that fits your foot and use it (TRS capable of course).