I have wondered many times how a human would fare if the kidneys dumped urine into the start of the large intestines somewhere about the appendix instead of into a bladder to be sprayed out. I’m assuming water would be reabsorbed and slower to process out, primarily through sweat and evaporation from the lungs, and maybe diarrhea, though it may be that other waste products, such as salts or ureas may be absorbed into the large intestines instead of being ejected, though I have no idea if it would, or if it would be ejected as intended. Do we have any biologists here that could give insight on if combining both waste paths into one would be advisable?
So the way evolution works, the design we have works well enough that it doesn't cause problems. It might be the best possible design or it might not, all that mattered is that whenever it arose in evolutionary history it was either an advantage over what camebefore in terms of survival so propagated or it was not detrimental and paired with something else genetically that propagated.
We can't definitively answer your question but we can speculate on why it's a good idea to separate urine and faecal matter. Urine is a reasonable medium for growing bacteria. That wouldn't matter in the colon but would matter if bacteria from the colon could ascend into the kidneys and diarupt it's function. Valves could help or a bladder that drains into the colon, but complete separation may just be better.
It may also be that the acidic nature of urine would disrupt the helpful bacteria we rely on to colonise our guts to help digest foods.
Another possibility is the constant flow of urine would mean our faecal matter would never dry out. It'd be like having diarrhoea all the time and we'd need to poop constantly. The colon retrieves enough water - but not all water - that's why poop isn't hard as rock. If it was flooded with fluid it may not need to retrieve fluid.
The fluid might even be stuck in a cycle between the colon and the kidneys and make it harder for the body to keep homeostasis - as the kidneys excrete more fluid to try and regulate fluid volume the the colon could just resorb it. Basically the colon could end up working against the kidneys and cause even more work for thenl body. It may just be less efficient than discarding water as needed.
Drier faecal matter in the colon and a reservoir of fluid in the bladder does also give us freedom to release when it is safe to do so, which may protect us from predators (having to stop to poop even a few times a day is dangerous compared to only going when you know it's safe to as there are more opportunities to be attacked by a predator). It would also be very easy to track an animal that leaves a constant trail of poop and urine uncontrollably behind it.
All or none of these may be reasons why we have separate urinary and alimentary tracts; it's impossible to know and would always be speculation. But regardless these do seem like reasonable reasons why we may have separate tracts.