so why not give upkeep and supplies to repair separately?
Because it's not useful. Supply consumption takes place as a whole. Any individual piece does not tell the player how many supplies they use or need.
How is separate information not useful? Having the estimate split into the different sources of consumption is much more informative for a player trying to improve their fleet. If your upkeep is high and you're having to turn back from expeditions too early, you might try to remove ships from your fleet, or add efficiency overhaul or get rid of increased maintenance ships or something. If you're losing a lot of supplies in combat, you might try to improve your fleet composition to take less damage, or deploy less ships. If you're losing a lot on storms, you can add solar shielding or more carefully avoid storms. Specific information is always going to tell you more,
and you can always just add them together to get total supply consumption . There's literally only upsides to having the information separate. Like you said earlier, you're never going to actually know how many supplies you need, so I think its much more valuable to get a sense of how many supplies go to different things. The total is almost certainly wrong because you will encounter unexpected circumstances, but the parts will give you a good sense of how much fights cost and how much storms cost which you can use to estimate how many supplies you need for events that haven't happened yet.
I also think it makes way more sense to tell the player about repairs/CR recovery as what they are: a one time supply expenditure not a monthly expenditure. You're not burning 400 supplies/month, you're burning 400 supplies once, over the course of this week. The game currently explains that by telling you that your current rate of supply consumption is elevated, but just telling you that you're going to burn x supplies to repair your ships seems much better to me.
Ship upkeep is the LEAST significant aspect of supply consumption.
My typical mid-late game fleets burn 8-12 supplies per day in upkeep (so 240-360 supplies over a month), typical battles burn 200-500 supplies and a single storm costs ~50 supplies (with a fairly big margin of error). I also have the choice of avoiding storms which saves the storm cost and increases total upkeep by taking longer. If I consider the supplies I get back from salvage after a battle, combat is usually net positive on supplies, and I often look for easy pirate fights to recoup supplies on long expeditions. Upkeep is definitely a very significant portion of my supply consumption, probably something like 40-50% of overall supply cost. But if I am exploring and not fighting, upkeep becomes my main expenditure. In the mid game, it is often what forces me to turn back from exploring.