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