The American founders didn't have good understanding of civil service type stuff back then. Coming from Britain there was a bureaucracy but if I'm remembering my history right it was mostly staffed by nobles who needed jobs and the overriding concern was that money should keep coming into the government. Especially from the colonies. This was actually part of the reason we ended up in a war for our independence. It may not have gone differently with a direct line, but we had to go through the undersecretary to the undersecretary to communicate with the British government. Which effectively made sure our concerns were never heard by the King until we petitioned him directly. Then he consulted his top advisor who also had not heard any concerns previously and they concluded the petition was worthless. To which we decided property destruction was the answer and cue the escalations.

So what our founders wanted was an independent civil service, but they had no idea how to make one. They only knew about patronage systems. And the one lethal blow to any patronage system is to say you can hold this position for as long as you want, as long as you're not corrupt. They knew it wasn't perfect. And they openly said we should be holding Constitutional Conventions on the regular to improve on things like this. For the record the two competing models are to lean into partisanship and hold elections, or run the judiciary as a technocracy with limited sovereignty. So the judges would actually figure out the supreme court and lower courts themselves in that system. Much like our military does now.

Both of those systems have their pros and cons but importantly, none of them stop determined ideological assaults on the institution. By the time you are hiring people it is too late to stop that. They've already been indoctrinated and they aren't going to tell the truth about it publicly. (For example all the judges that overturned Roe v Wade, said it was settled law or something similar in their confirmation hearings. Then they flipped the literal second they had the majority on an abortion case.) You have to stop indoctrination at the source, in education. Which is why there's such a huge push by conservative Christians to destroy public schools.

Anyways that's probably more than you wanted. TL;DR is it was the best system they had at the time, and they could not have foreseen fuckery like capping congress which obliterated the idea of actually representing the local views in a national body.