The FP of foot infantry with basic, localy avalable weapons is so low that it would take scores of regements to rate more than the 1FP for millita, so the need for a mech factory basicly represents the need for infantry to have more than just sturdy boots. They need medium and heavy weapons, APCs, jeeps, artilery, and all sorts of other fun stuff that takes real work to put together.
An older set of rules set the FP of basic foot infantry as zero (0) so that you probably could set your infantry as just being farmers with what rifles and shotguns they had on hand anyway, and get exactly what you pay for, but I don't know that that still holds true. For properly armed infantry with heavy weapons that can genuinely fight against mechs, that is to say that rates some number of FP, you need something more.