Well the voyager probes were designed to visit planets, not just be flung in to space. Their very path away from the sun involved a ton of planetary passes. Oh and the probes are pretty big and heavy.
What has been discussed is the creation of small probes designed just to be flung in many directions. We just haven’t felt the need to prioritize that over other possible missions. My hope is that this is something we will do in the next couple of decades as space travel becomes cheaper.
There's a lot of reasons we don't do that. To start there's limited value to be had just throwing them out in all directions. The Voyager probes were able to do a lot because they were on very specific paths that visited a lot of planets and was able to use them to get a boost at the same time to reach the incredible speeds they got to.
Next technology has advanced really quickly so by the time one is designed, built, and tested there's way better technology we could include in terms of cameras, computers and sensors so there's little incentive to use the same instrumentation over and over again because we can always pretty much always get better science out of newer equipment. On top of that there's limited returns on returning the same equipment to the same body because we've already gotten most of that data before.
Next if we're talking about surface probes they're very specific to the location they're going to and the science we want to perform there. For example until we made Curiosity most of our rovers were solar powered which doesn't really work much further out in the solar system like on Jupiter's moons. The lack of atmosphere on many potential destinations is also a big problem because it massively limits the size of potential probes because they also have to carry their own landing fuel all the way out there.
Finally because of all of that there's not really the money for launching missions that won't probably yield that much new science. They'll return a lot of data to be sure but there's limited new insights because the existing data already gets analyzed and examined to within an inch of it's life to wring as much information as possible. I do agree we should be spending a lot more on planetary exploration but there's a lot of reasons each probe/rover we send out is largely designed and built from scratch.
It’s not really practical. For one, that’s expensive. In addition, our space crafts are becoming increasingly better designed. Eventually, as in 50 years, it’s increasingly likely we can do your suggestion at 1/1000 the cost with more sensors. It probably makes more sense to invest in developing future technology than doing your suggestion today.
