Nick - MV & HV terminations experience much greater electrical stresses than a LV termination, at least given the amount of material & space the termination must fit into to. The cost of failure of MV termination is also likely much higher than a LV one. The total damage or safety risk is going to be higher for a given high voltage termination than one at a lower voltage. For those reasons an engineered product makes a lot of sense. Also, from an operations standpoint, an engineered product that is quick to install and is reliable is preferable at medium voltages. Utilities that either need to take an outage to install (or have an ongoing outage and need to repair) a termination are willing to pay the high upfront price of these products in order to get lines back in service and customers back on.
Skirts/sheds/ribs are an affordable way to increase the total distance or 'creepage path' between energized components and ground. They reduce the total liner length a termination needs to be by adding folds. You see this same application when looking at medium/high voltage insulators on overhead lines. Do a little googling on creepage path and it will become apparent why the ribs are added.Their physical construction also helps diver rain/dirt/debris off the the termination, preventing ingress into the cable, at least to a degree.
