We may not have the capability to build a Dyson cloud around our own star (yet), but we do already have the technology. You could do so with 21st century tech, and the ROI would be positive.
The really advanced civilizations developed their technology just enough so they could build a really safe and reliable Matrix to live out their lives in.
They would still consume their local stars to extract and store the energy so their matrix can survive for trillions of years, rather than a few million or billion.
1. disassembling the stars to make them live a thousand times longer
2. it only takes one expansionist civilisation seeking interstellar exponential growth — even if 99.9% of civilisations are quiet, that's just 3 digits of the 22-digit number of stars in the observable universe.
> disassembling the stars to make them live a thousand times longer
Why would they do that when there are insane amounts of start with billions of years of life ahead of them. What purpose would that serve at this point in time?
Already today, having passed peak-child a few years ago, we can see that there are futures possible where humanity doesn't grow. I.e. humanity might be fine advancing with a fixed amount of power.
In that case, the stars are not 'waste'.
Historically, this idea of growth as method for advance is only a few centuries old. It's not hard to see its end as efficiency keeps increasing.
Which seems to be the most likely explanation to me.