This question, to my knowledge, is only applicable to Rocket Lab's Electron, pressure-fed engines that use non-hypergolic fuels, and other engine cycles that don't use a fuel/oxygen-rich preburner. My question is why can't someone like Rocket Lab store both LOX and RP-1 together in one common tank in a premixed ratio? I understand in something like a Falcon 9 it would at least be necessary to supply more fuel to the preburner but in something like the Electron that uses an electric turbopump that requires no preburner why couldn't both the oxidizer and fuel not be stored together? Intuitively, this would remove the hefty weight of tank bulkhead(s) that divide the oxidizer and fuel tanks and make your injector(s) much more simple.
As Organic Marble hints, there is about 140 degrees Celsius between kerosene's freezing point and oxygen's boiling point; there's no temperature at which both are liquid.
Even if the propellants were more thermally compatible, putting your fuel and oxidizer in the same tank is a really dangerous idea. Typically, propellant tanks are pressurized with helium or nitrogen -- nonreactive gasses -- so that any spark in the tank won't start a fire. With both oxygen and kerosene readily available in the same space, the tank is a bomb waiting to go off.
While mass ratios are very important to overall rocket performance, safety is more important.
Because it will almost certainly go KABOOM.
Intimately mixed fuels and oxidizers are pretty much indistinguishable from explosives, and in particular, LOX intimately mixed with flammable hydrocarbons is wildly dangerous -- rather than being something you can handle, it tends to be set off by shock, vibration, or adiabatic compression that can be caused by closing or opening a valve.
Perhaps the most infamous LOX-hydrocarbon fuel mixture was LOX and liquid methane (which are perfectly miscible), discussed in Clarke's Ignition -- it was supposed to act as a monopropellant as you are describing, but was allegedly (though disputedly) so touchy it could be detonated with a bright light.
At least one of the Bell X-1 rocket planes was destroyed when LOX in contact with a oil-impregnated leather gasket detonated due to vibration. A bulk mixture of LOX and kerosene fuel is far worse -- it surely could not survive passing through a turbopump, and there would be little to stop the detonation in the combustion chamber racing up through the fuel lines and setting off the whole tank. (Detonation arrestors exist, but they don't save your engine and fuel lines from being rapidly disassembled, nor do they protect your tanks from shrapnel, as Clarke learned).
Additionally, RP-1 is going to freeze at LOX temperature. The interaction of small particles of hydrocarbon probably won't help matters.
to put it in 3 words "it will explode." you see fuel burns with oxidiser and mixing the two in the same storage will well let me make it in steps:
you ignite the engine... the flame flows into the tank... that burns too... in a closed space... rapidly... which causes the tank to burst... and then you have no more rocket. ):
The history of rocket fuel development has been dominated by getting them to work at the range of temperatures we want. For instance finding a fuel that is liquid enough to work in the Arctic, and not so volatile that it can't be stored at reasonable pressure in the desert.
Unfortunately, kerosene freezes well above the boiling point of oxygen. It might be possible to turn it into a slush in oxygen, with development, and admixtures to stabilise it. After all, various powdered metals, boron, aluminium, beryllium (nasty exhaust!), have been added to fuels to increase the heat of burning, with various gels and black-magic to keep the mixture liquid. However, the history of monopropellants has been very much what you'd expect from trying to store what is basically a liquid bomb.
Read Ignition for one man's on-the-spot story of the development of rocket propellants.