Why did the founding fathers make it so difficult to amend the constitution?

 This is a useful bit of history because it places the driving idea on the table.  They were correct that no traditional form survived for long and all this around the French Revolution.

The Doge elective process acknowledged the central position of wealthy families but then made interference impossible and also able to pick an able man.

Obviously an able individual could be promoted into this central position simply because he was obviously able.  That was good enough and ended coups and counter coups.  god knows the families likely fought like cats.

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Why did the founding fathers make it so difficult to amend the constitution?

Because the goal of the founding fathers was not to optimise the new state towards good governance, but longevity.

Venice

See, they noted that European states often had a problem of longevity and stability. The autocratic monarchies of France, Austria and Russia were unstable, prone to civil wars, foreign claimants and more. They would have known of the Hundred years’ war, how one mistimed death made Russia nearly collapse in early 17th century, how a rebellion tore their own England apart in 17th century and so on.

Yet they also knew there was a government in Europe that was stable and long-lasting. The Republic of Venice was founded 697 C.E., before Charlemagne's grandfather was even born. That republic stood and withstood the test of time, being over 1000 years old with no sign of decay. One of the reasons why Venice stood so long was a long-winded, complex, but ultimately fair system of electing a new Doge. It went through a few iterations with minor changes, that’s who it worked since 13th century:

Heads of 30 of the most powerful families are assembled, their names are thrown into a pot.

From the pot, 9 names are drawn at random, the other 21 are discarded.

These 9 people draw up a list of 40 names.

These 40 names are thrown into a lot, 12 names are drawn out, the rest are discarded.

These 12 people draw up another list of 25 names.

These 25 names are thrown into a pot, 9 are drawn out and the rest discarded.

These 9 people then draw up another list 45 names.

These 45 names are thrown into a pot, 11 are drawn out and the rest discarded.

These 11 people then draw up a list of 41 names.

These 41 people elect a new doge.

The system is ludicrous, but it basically ensuers it cannot be rigged. There will be 41 electors at the end who will have the authority to choose the new ruler of Venice and they will come from the most powerful families of Venice, but no one, no matter their power, wealth or status, will have any outlandish role. There’s no way to influence the process until it is towards the very end and the final group of 41 people aren’t indebted to anyone. The final result may not give you the best, but you can rest assured it will give you someone acceptable and that’s good enough for longevity. The system might survive to this day even, were it not for French revolutionary armies who ended the Republic by force of arms.

This was what the US founding fathers wanted: a state that could exist in perpetuity, and if it reduced the quality of governance, that was an acceptable price to pay.

It should further be noted the US Constitution would be fine, were it not for industrial revolution. It’s an 18th century document that solves 18th century problems, most of the issues with the US Constitution have have to do with the fact the 19th, 20th and 21st century saw mankind advance faster than it advanced since at least the invention of agrictulture. If you need a week to send a message from Savannah to New York, states’ rights make a whole lot more sense than if you can fly from one to the other in a couple of hours.