That's because there so much of them and most of the range is not very comfortably accessible (you usually need to endure some discomfort either getting there or staying nearby).

Where you can get nearby fairly easy like base Las Torres, you'll have plenty of people doing just that.

I don't think this is by intent. It's just that the countries themselves are relatively poor and have higher priority projects for their limited resources.

The higher Himalayas are largely unpeopled as well, especially above 4000m. The only time I met people above those altitudes were at night in camps/settlements. I'm sure the EBC route is more crowded, but that's one very small trek in an enormous mountain range.

The industry that has sway over the Andes is mining. Against that, tourism income is negligible and in fact more tourism could threaten the mining industry.

I did Cerro El Plomo in January and it was a very blissful experience. Barely any people and no phones. Can highly recommend.