It really could have been 1904 when prepared, fully ready to use mustard was first created by a corporation. It needed to be able to prevent separation in storage and be acidic enough to withstand infections from food poisoning. Use a little vinegar and hydogenized fats, and you have a store bought condiment.
However, making your own mustard just before you need it, isn't exactly hard. wisk some powdered seed into some warm water or vinegar, and voila. But, going into the 20th century, corporations were on their way to making people think that cooking was hard, so hard, that why not buy some premixed cornbread meal, or biscuit flour, just add water. Measuring out flour, baking powder, and sugar is just too hard for this generation. Why take your own corn to get it ground? Why buy and measure out stuff you buy in the produce? Just buy ours. It's the best.