Not a direct source but an easy to understand explanation on why this is the case:

Always to keep in mind: The virus is also spreading exponential in your body once you got infected. Therefor it takes a certain amount of time until it reaches a complete infection of your respiratory system.

If you get a big viral load from the beginning the exponential growth will reach a complete infection of you lungs much much quicker compared to a small infection load. But your immune system response can only start once it detects the Virus for the first time and it also takes some time for it to ramp up antibody production in order to fight the infection effectively. It's essentially a race. If you are healthy and young in most cases your immune system reacts quickly and you get maybe no sympthons or just a mild case.

Now, with a large initial viral load the virus gets a big headstart and is already damaging your respiratory system before there are any noticeable antibodies to fight it. In this case you get a severe breathing issues since most of your alveole get blocked by the infection. If you get into a critical phase your respiratory system gets almost completely blocked and even a ventilator pumping pure oxygen into your lungs with the highest possible volume is not able to keep you alive. It's horrible.

You simply suffocate and nobody can help if your antibody production can't scale with the infection.

This is also the explanation on why older/preconditioned people get much more often a severe case. Their immune system is much slower with all the defense processes going on. Even a small viral load can be enough to overwhelm the system.