Why would "the lawyers" choose to conspire about this in particular?

According to BLS [0], "more students graduate from law school each year than there are jobs available." Sure, part of this is a scam by law schools, who have convinced young people to take big student loans with unrealistic ideas about a career in law. New law schools open all the time, and it's not in order to address a shortage. Mostly, however, lawyers are like the apocryphal firefighters who light their own fires. They need business, and they run the system that determines how much business they get. All of the following questions have only one answer: should such-and-such activity be regulated more closely? does such-and-such crime warrant a more severe punishment? should such-and-such prosecution not involve a jury of peers? should such-and-such type of legal process take longer and be more complicated? Yes, yes, yes, and yes! Who decides these questions? Mostly, judges and lobbyists.

Also, using words like "conspire" or "conspiracy" is a total straw man. When lots of people have the same interests, e.g. increasing the number of times an average citizen must hire a lawyer, they will act in similar ways without needing to communicate. Seeing conspiracy where no one suggested one is a way of ignoring the point.

[0] https://www.bls.gov/ooh/legal/lawyers.htm

Why do Americans elect local judges, but not Supreme Court judges?

It's a terrible idea either way. People usually vote for local judges knowing only the candidate's occupation (since it's indicated below their name). Doing research on candidates is hard work, especially local ones you can't find on YouTube, and few will consider leaving a choice blank if they don't know who to pick.

Supreme Court justices are appointed to lifetime positions specifically to give them the freedom to ignore politics and use their literal best judgment (if they can make it through congressional approval). What kind of supreme judge gets subjected to judgment every few years?

Supreme Court judges are not elected because they’re federal judges, and no federal judges are elected. It has nothing to do with how “supreme” they are.

The US has fifty different independent justice systems run by sovereign, independent States. Each of those States has the power to appoint judges in whatever manner it wants, so of course you get some variety. It’s not like “America decided” to have local judges be elected.

No, Americans decided (via their states) that they wanted to elect judges. Why? Do you elect teachers? Firefighters?

Back when this was originally decided, most Americans did not have the right to vote. It would be nice if we could reconsider decisions like this now that we do have that right, but not only because we could come up with a different way to hire judges...

No, not all States have elected judges. Only some do. I agree that electing judges is idiotic so I'm not sure what you're trying to convince me of.