Instead, it is so entirely overgrown with a thousand years of vegetation, that it is difficult to spot the difference between a hill and a temple.

I was in tikal recently (drove from Flores, as you pointed out), and it was astounding to compare the parts they have excavated and restored to the rest. You would see hundreds of small hills and know that under each is an undiscovered building! Had it not been for the escavated parts, I wouldn't have even realized the hills were anything else.

As a parallel, under parts of Guatemala city (far to the south in the highlands) there are the remains of a Mayan city from pre-classical to early classical. The center of which is under a high end shopping mall. There are similar hills there covered in grass in a nice lark. I have laid on hills like this throughout the city without really realizing there were ancient buildings underneath.

Thus, there is a good rule in the capital. If there is a big, grassy hill, it is probably because there are ruins underneath. The government prevents people from developing that land, but no one has the money to excavate. Thus they stay entombed, waiting for future grad students and funding.

How many people who don't wear a uniform use a 100ft rope to enter or exit a jungle canopy if they aren't in immediate fear for their life? You're underplaying the logistical difficulty.