The planets needed to be in a certain alignment for the Voyager launches to work out like they did. The slingshot only works in one direction (if you go backwards, you lose momentum). Maybe we haven’t seen another such alignment since, or didn’t have a mission ready to launch if we did.

Space travel is very expensive and NASA has a very small budget these days.

Back during the space race, NASA could afford to launch multiple missions per year. Now they can barely afford to maintain existing missions and are lucky to launch a major missions every few years. Which is why they’ve moved to buying space on commercial missions, as it’s cheaper to only pay for a spot on a rocket/craft than to pay for the whole thing.

NASA also has to justify its missions to congress. Sending rovers to mars and probes to the moons of Jupiter and Saturn have actual scientific interest and can answer questions about the formation of the solar system, and the viability of life off of earth.

Slingshotting something really fast sounds cool as fuck, but there’s not much data to be gathered there. We’ve also recently beaten the “fastest man made object” record with the Parker Solar Probe, as it’s currently whipping around the sun at ludicrous speeds while it collects data about the solar atmosphere and magnetic fields. It’s moving a lot faster than voyager ever did, as it needs an insane amount of speed to orbit so low to the sun. It’s actually much cheaper, fuel wise, to travel to Pluto than the sun.

So why waste billions of dollars to fling something out into deep space? We have barely even seen all Of the celestial bodies in our own star system, and there’s not much to be learned about the empty vacuum beyond the sun. The only justifiable reason would be to send a probe to another star system entirely. But that probe alone would have to be the largest, most expensive space craft humanity has ever built. It would need to be able to power itself for centuries, have a communication system capable of sending data over interstellar distances, and likely need a way to autonomously harvest its own fuel, as there’s very little point in sending a probe screaming past Proxima Centauri and taking a few hazy pictures of planets as it goes. We’d want the probe to be able to stay in and explore the new star system, and the only way to do that is to have enough fuel to move around an entire system, or create more fuel as it goes. Something like that has never even been tried before, and the risk is high when you won’t know if it worked or not for a few hundred years.

Especially seeing the original flight plan was for 5 years.

Or 17776

Or the one the Klingons destroyed for target practice.