I was thinking something similar too:

Why almond/soy milk is more expensivs than cow milk? To make almond/soy milk you just need to maintain the plants. To make cow milk you need to have cow food, take care of the cow etc. and generally it seems much more inefficient than making plant-based milk. I dont know if they use the whole almond/soy etc, but still.

On this wikipedia article, under Nutrition and Sustainability, you can see for example that while Soy and oat milks have around half (almost has around ¼) the calories of cow milk (plant based milks seem to be healthier too, due to lower saturated fats and sugar contents), they are much more efficient to produce. Like, oat milk seems to be around 6-7 times more efficient to produce than cow milk which counters the fact that it may have half the calories.

Some may consider cow milk as byproduct of the meat industry, but since baby cows need to feed from it and cows already spend energy to make, I dont think of it that much as byproduct, it’s necessary.

As someone people said though, it probably is due to the financial incentives given to the cow milk industry🤷