I think we can all agree, first of all, that a satisfying dump really is one of the purest joys in the world. And while deep in thought, appreciating the natural marvel that is pooping, we wondered just why pretty much every poop you've ever created in life is always accompanied with some urine.
Turns out there's a legit reason why you can easily pee without shit betraying you, but not the other way around. Go on, try taking a shit without letting some pee slip out. That shit ain't possible, fam. Literally.
The logic behind urine tagging along with stool is pretty simple. There are many muscles, voluntary and involuntary, that control the passing of our bodily wastes. Both the urethra and the anus have internal and external sphincters to moderate this passing.
The internal sphincters for both these parts of the body operate involuntarily - relaxing as and when the pressure builds up. It is the external sphincter that is an entirely voluntary muscle that one can relax at will. So you can tighten them and hold in your pee or poop, only releasing bodily wastes when and where you choose to. Well, most of the times.
The external muscle that we relax when peeing, however, is a lot smaller and easier to isolate than the muscle relaxed when pooping. So when you decide to take a leak, you're able to do so without the general musculature of the pelvic floor relaxing.
When pooping, however, apart from allowing solid waste to pass, the relaxation of the considerably stronger anal sphincter also decreases tension in the weaker urinary sphincter, allowing urine to pass simultaneously. Causing some pretty glorious relief, no doubt.
In a nutshell, it's a fight between the shit and piss muscles and shit trumps piss every day of the week, and twice on Sunday.