Buyers pricing a tripod system are often surprised that the head costs more than the legs supporting it. The legs are a solved mechanical problem; the head is not.
Legs are tubes with clamps
A set of legs must extend, lock and stay rigid. Those are well-understood requirements met with straightforward materials and geometry.
Material choice separates the price bands more than engineering does, with carbon fibre buying weight reduction and vibration damping rather than any new capability.
A cheap set of legs is heavier and less pleasant to use than an expensive one, but both hold a camera still, which is the requirement.
A fluid head has to resist movement smoothly
The purpose of a fluid head is not to move freely. It is to resist movement by a constant amount, so an operator can start and stop a pan without a jerk at either end.
That resistance comes from a viscous medium in a sealed cartridge, and the quality of the result depends on how precisely the surfaces are machined.
Small tolerance errors produce uneven drag that shows on screen as a stutter at the beginning of a move, which is exactly what cheap heads exhibit.
Counterbalance is a spring problem
A camera tilted forward wants to keep falling, and a counterbalance system applies an opposing force that varies with the angle to cancel that tendency.
Matching the force curve across a range of camera weights requires a spring mechanism with adjustment, which is a significant part of the cost of a good head.
Heads without it work only when the camera is perfectly balanced on the plate, and any change of lens shifts the balance point and reintroduces the problem.
Temperature changes the fluid
Viscosity varies with temperature, so a head damped correctly in a warm room becomes stiff in cold conditions and looser in heat.
Better cartridges are designed to reduce that variation, which is one of the less visible differences between price tiers and one that only shows up on location.
Operators working in cold climates often warm a head before shooting for exactly this reason, because the first pan of the day is otherwise unusable.
Where the money is genuinely wasted
A head rated far above the camera's weight is damped for a load that is not present, and light cameras feel stiff and unresponsive on it.
Matching the head's payload range to the actual rig, including lens, monitor and cage, matters more than buying the strongest option available.
Legs are the opposite case, where buying beyond current needs is reasonable, because a heavier future camera will still sit on the same set of tubes.