Some people are just scared and do not want to get into problems. And you know how screwed up officials of these countries are. So people usually want to keep it simple, obtain a visa and avoid the hassles later on.
To obtain pakistani visa is insanity, because they just try to rip you off. Just imagine if you are a family of 3 kids. Anyway... Pakistan also issues a National Id card for foreign nationals for 10 years, which you can carry with your US passport without obtaining visa. However that card itself is so expensive that you are better off obtaining a US visa if you just want to travel once or twice. Another alternative is to have a pakistani passport and a US passport for traveling in both directions. It is absolutely normal and I have seen Canadian & British citizens using dual passports. (I think some people are also scared of US goverment for using two passports although its normal)
Regarding Indian dual citizenship, this is very new and they havent finalized it. Read the article carefully. It says
"Although the Bill has been passed out the rules, regulations and requirements are still under process."
The bottom line is that to indian/pakistani people, US citizenship is too important and they are willing to spend money on visas rather than answering questions to un-educated officials. That is why people do not follow the easy path.
best,