Put yourself in a soldier's boots.
A huge battle is going on in a ruined city. You're inside a building and faced with a long hallway with many rooms on the left and right. Anybody could pop out at any moment so you need to be ready. Your bolt rifle has 3 rounds left, one is chambered.
Do you:
A) Stop in the hallway and eject one round to load 3 more, leaving yourself wide open to attackers for the amount of time it takes to half-load your weapon and place its remainder back in your pouch?
or
B) Keep that round chambered, fix a bayonet, and have your finger on the trigger and ready to take out whatever gets in your way? Instead of sacrificing precious time for an extra three rounds you'll likely not be able to use in close quarters anyway, you can reload at a later point once the situation calls for it. The magic of RO's manual bolting.
Say you chose option A. In game terms you more or less engaged in a longer reload sequence. Interrupting it (in a perfect world where this is possible) would leave you with 2 rounds, should someone show themselves. Should you get the chance to finish, you have five rounds, but what advantage does this really give you? Your bolt action rifle is good for one, maybe two misses in a perfect close quarters scenario. If you can't stop a confrontation with two rounds you probably didn't have a good chance in the first place.
So option B must be the more timely and overall sensible choice. You have your 3 rounds, and most of all you're able to focus on the situation and not managing your ammunition. You also won't have a half-loaded clip in your pouch. Once you've spent your 3 rounds you can reload as usual and get back to the fight.
"Topping off" bolt action rifles is, in my opinion, foolish to do anywhere but in a quiet situation where you are not in immediate danger. And in RO, that situation is almost never present unless you're sitting at the edge of the world taking shots at pixels in a field. The process would be longer and would cause people to lose track of exactly how much ammunition they have left. It would only serve to complicate a situation that really can't be helped anyway.