As it's been pointed out by some comments in the past, the reason in the title is because the game is coded to only use one stat per building function, if a building's function uses any stat at all.

It's possible to change what stat is used, hence this mod's existence, however, it does not seem possible to have it affect more than one at a time without decompiling the game's code and making changes this way. It is prohibited to make mods publicly available which have such changes.

I personally have fiddled with it and have tried adding a secondary stat by changing the fcs.def file which unfortunately does not seem to make any effect.

All of the processing buildings (making iron plates, steel, building materials, etc) raise labouring, so it's not impossible to continue to train this even with this mod enabled.

If you really want to train your strength with a method alternative to filling your character's pockets with rocks and running about, there are other mods that add buildings which train that stat. I have one such mod uploaded to Steam's workshop but the purpose of this discussion was not to plug my mod.