Legal and criminal responsibility for actions while governing leads to a situation commonly seen in a number of republics, most prominently ancient Rome, which the creators of the American constitution desperately wanted to avoid. While in office, you're safe to some degree with the powers of the office. Once out of office, you're an immediate legal target and all the lawsuits get filed. Solution? Don't leave office. Avoiding that is the basic logic behind immunity in political office; stable transfer of power breaks down when you know your successor is going to imprison you.

My guess would because the president is also technically the chief law enforcement officer? Not saying I like it, but that’s how it is I think.

Impeachment is what exists to hold presidents accountable, and then voting is what holds the senate accountable, when they choose not to hold the president accountable.

I think the president can be prosecuted for criminal offenses though when they’re out of office, since what I said above would no longer apply.

This is all correct. The issue is that we've gotten in the mode of not prosecuting our political enemies once they're out of office. That's actually a very important political precedent that has largely served us well. The peaceful transfer of power from the Federalists to the Democratic-Republicans was a huge deal in 1800 and it guaranteed that subsequent transitions (mostly) followed the same model. The problem is, there were no tripwires: what was too far for a president to go? Ford pardoned Nixon, which may have been the right move at the time (I'm mixed), but it became mixed up with the peaceful transfer of power tradition and basically was interpreted as a free pass.

I think we may be about to see the the end of the tradition of get out of jail free cards for former presidents, but I don't know what the long term consequences will be.

Because they /all/ do it. Every single president. To prosecute looks political but more importantly if the current president prosecutes, they'll be the defendant next time the other side is in and they'll be guilty as sin.

Why do they all do it? Might be a better question to ponder.

If every president (or other leader, this isn’t just about the USA) genuinely needs the power to do certain things which are against the constitution of their nation, the correct solution is to change the constitution to reflect reality.

If a government allows its employees to get away with doing things which are against that government’s own rules for their own operation, then a few bad apples spoil the whole barrel.

If you need government spies secretly looking on certain citizens without the targets being aware of it via a warrant, make sure they can’t ever be used like Nixon. If you need government assassins, you definitely don’t want them used the way Nixon used government spies.

Just look at the (absolutely absurd) list of presidential pardons each has performed on the way out...adds lots of color to this story.

the cynical take is: that’s the whole point of the presidency in the first place. Have one guy who is technically in charge of the machine, the machine commits countless crimes, then after 4/8 years you pin all those crimes on the single guy you selected, go “whoops! bummer about the drone strikes!” and then pardon him.

What is "unconstitutional"? It's what the Supreme Court says it is.

The Court has had the opportunity on multiple occasions to rule that this domestic spying is unconstitutional, and has declined to do so. Ergo, for all practical matters, it's legal.

Everyone in office takes an oath to uphold the constitution. The Supreme Court gets the final say, but it is still the responsibility of everyone to do what they know to be correct.

Nothing in the constitution says that the Supreme Court has that power.

To gain the presidency it is all-but-mandatory to have the enthusiastic backing of a big chunk of voters and the tacit or explicit approval of around half of them. Combine mass popular support with executive power and there isn't a lot practically that can be done.

Furthermore it appears that most of the US politicians have some sort of borderline-corrupt activity going on if not being actively involved in something illegal (Epstein springs to mind, various scandals that turn up around presidential elections). People in glass houses are cautious in their stone throwing.

And a final and decisive stroke - there is plausible uncertainty about whether the action is illegal. These are people who write the law, they are not expected to be stay-inside-the-legal-lines types because they all have different opinions on where the lines should be and are usually in the process of drawing them. They appoint the people who determine what the words mean and that leads to occasional creative reinterpretations. There isn't time to prosecute over every detail that someone objects to given the range of objections there are.

Because the AG works for the president and can be fired for any reason

Because it sets a bad precedent. The former used his presidency to push as many buttons as he could to see just what the extent of the President's powers are. Sure there are some legal rumblings now, but I'd wager he will never see a prison. Ever.