weren't going to vote for Trump, Trump was not yet even being viewed as a legitimate candidate by half of the Republican party base and had not locked up the nomination yet (let alone running as an incumbent with a 4 year record), and so on.

A better comparison would be polls from March/April in 2012 where Obama was ahead by 11 points because, you know, he was an incumbent president and the other party hadn't yet consolidated around Romney as his opponent. If Obama had been losing by 6 points instead, hard to see him getting re-elected. Same is true for most incumbents who ran for re-election, other than Carter who lost.

The typical pattern is for an incumbent to start out with a big lead that gets chipped away. It's hard for someone who has been POTUS for 4 years to win over new supporters. And if you're starting out in the toilet, and in this case are also a giant steaming turd, good chance America just wants to flush and light a match.