Why don't they make more sprues?
The short answer I learned when Target Games went into producing minis for the various Mutant Chronicles games in the 90's is as follows. (I worked as an editor at that company for some years.)
Sculpting a master for either plastic or metal has a relatively low initial cost. Making a mold for metal is also comparatively cheap, but as soon as you have the molds you produce the minis practically by hand - you can only cast a small number of figures per day. It's fairly easy to recover the production costs, as you don't really need to sell a ton of figures. This is why smaller companies can afford to produce and market lead miniatures.
Cutting the tools for a plastic sprue is (or at least was in the 90's) very expensive, and this is the major cost when producing plastic minis. Once the tools are complete you can use them to produce huge numbers of sprues, and each individual sprue is ultra cheap in itself to produce, but you need to sell a really large amount of plastic in order to recover the cost of the tooling.
If anything remarkable has happened within the production of lead and plastic minis, please feel free to update me on the subject!