I've never understood why they have tapers, must be a past law along the line somewhere?
I'd doubt highly there is a law on the books requiring ball joint / tie rods to have a taper.
However, there are some advantages.
1.) You end up with something that doesn't easily interchange, so you don't have people tempted to put it back together with hardware that isn't up to spec. For example, you don't have some shadetree mechanic pull out a grade 8 or better bolt and replace it with a rusty grade 2 or worse bolt, and then kill somebodies 6 year old because it broke at precisely the worst possible time.
2.) It forms a really tight joint, and is pretty easy to manufacture (with the correct reamer and other hardware at your disposal). You get an oversize hole that is fairly easy to put together, and when done right, there is zero play, even if the fastener gets a little loose. With, say a bolt and a spherical bearing or a straight shank ball joint, you have to have really really good tolerances on the bolt / stud and hole to get zero play to start, and if the fastener gets loose, it's going to "egg" out the hole that the bolt passes through quickly. But, the tapers are definitely harder to take apart, though, so there is a price to that convenience.
3.) From a component standpoint, a taper type ball joint is a pretty robust thing. The stud doesn't have any big diameter changes, so there are no stress concentration factors, and therefore no locations for cracks to start from. A straight style ball joint really is going to need a shoulder to bolt up against (in the stud), and that's a location that is sensitive to cracks, and you're loading it two ways (both axial loading from being torqued up and bending because of the suspension loads trying to snap it off). This would make for a much less robust system. A bolt and spherical bearing is doing things to a bolt that it really isn't designed for (they're designed for tensile loading and shear strength, but bending is not really an approved usage, not that we don't all do it.) unless you've made provisions so that you've got your bolt in double shear. I think there was a provision in the Formula SAE rule book that said if you use a spherical bearing or rod end that it must be in double shear or at a minimum captured)
Not sure that answers your question, but there are definitely some advantages to using tapers.
And yes, it is effectively just a big nut and bolt doing a job, but it is a VERY important job in most people's view.
BTW, Speedway has taper ball joint reamers available.