Why must conduit be fully assembled before the cable is pulled, rather than built up around it?

There are two reasons and the second is the one that makes it a rule rather than a preference.

Reusability. A conduit system is meant to be a raceway you can pull new conductors through later — that is much of the point of using it rather than cable. If the wires were laid in as the run was assembled, there is no guarantee the path is actually pullable. A joint that was fine to slide over a wire may have an internal edge or an offset that will not let anything through afterwards. You discover that in ten years, when somebody needs a new circuit and finds a conduit that cannot be used as one.

Damage you cannot see. Assembling around conductors means sliding fittings over insulation, threading joints beside it, and possibly cutting or reaming with wire present. Every one of those can nick insulation in a place that is now permanently enclosed. A nick inside a metal raceway is a fault waiting for vibration or heat, and there is no way to inspect it.

Pulling afterwards, with proper lubricant and a smooth complete raceway, is a controlled operation where damage is far less likely and far more likely to be noticed.

There is a third reason that comes up in practice: you cannot verify the run is complete and correct once wires are in it.

A finished empty conduit can be checked — you can look through it, push a fish tape through it, confirm every joint is made up tight and that reaming was done. With conductors in place, none of that is possible, and the joints that were assembled last are the ones nobody looked into.

That matters most in metal systems, where the conduit itself may serve as the equipment grounding path. A joint that is mechanically together but not electrically continuous is invisible with wires in place and is exactly the sort of thing this rule prevents.

It is worth adding that pulling is not as hard as people fear, and the reason the rule survives is partly that the correct method is genuinely easy once you have done it.

Fish tape or a pull string left in during assembly, proper pulling lubricant, and a second person feeding at the far end. A long run with several bends needs an intermediate pull point, and the codes limit how many bends you may have between access points for exactly this reason.

If a run is so difficult to pull that assembling around the wire seems attractive, that is usually a signal that the run needs a pull box in the middle rather than a different technique.

One thing worth saying about rules of this type generally: trying to reverse-engineer the single reason behind a code provision is a losing game.

Provisions usually accumulate several justifications over decades, and finding one reason that does not apply to your situation is not permission. If a particular run genuinely cannot be done the normal way, that is a conversation with an inspector before the work rather than a decision made on site.