Why don't we include loan payments in NPV ?

I think the fundamental misunderstanding you have is that you think that Cash Flows from Financing Activities includes interest payments. It does not. In only includes principal repayments. Cash Flows from Operating Activities does include interest payments. Look at any Income Statement + Cash Flow Statement on any 10-K from sec.gov and you'll see this to be true.

As Charlie Munger says, "I've never heard an intelligent cost of capital discussion".

Cost of capital can mean two things, and it's often not clear which definition people are using. Cost of capital can mean:

how much it costs you to borrow money (e.g. 8% annualized interest rate to borrow $1m with 10% outstanding principal repayment every year)
the opportunity cost of deploying your capital into whatever you're calculating NPV for (e.g. 9% historical nominal return from the S&P 500)
The discount rate matters for the second. It doesn't matter for the first.

Now, all that being said, to calculate the NPV of an investment, all that matters is how much cash you outlay initially and how much you're getting back at each period of time. Therefore, you are correct that debt repayment, in terms of both interest and principal should be taken into account when calculating NPV.