Yes. And because it takes years to figure out the science part. The industry that produces the rest of the spacecraft is set up to also work on year long timelines. Producing the thrusters for a satellite in a few weeks would just be a waste of resources. You would spend most of the year without a job.

It is possible to do things faster. If things are truly desperate I am sure the US military's would just snatch up every satellite engineer they can get ahold of and give them blanket checks to whatever they need to manufacture a probe as soon as possible. But probes require a lot of highly specific components. And if only one of them is missing the entire program grinds to a halt.