Why would they even do so ?
Large ISPs are public, so this activity would appear as extra revenue (if they sell traffic data) in their financial reports and annual reports.
The most likely is that ISPs are just respecting the local laws, and doing the minimum retention as required by the law (because more data storage = more costs),
and that their actual fear is that someone leaks this data and causes reputation damage, so they'd avoid storing anything if they can.
ISPs are also in the business of analytics [1, 2], and a significant percentage of customers hiding their traffic reduces the value of their analytic products.
This view is extremely western, not all ISPs are obligated to show "financial reports", and "shady analytics" does not imply a user's complete network traffic record into perpetuity. And even if your arguments were valid, this is not limited to the ISPs financial gain, but surveillance which occurs in every country.
The most likely is that ISPs are just respecting the local laws, and doing the minimum retention as required by the law (because more data storage = more costs),
and that their actual fear is that someone leaks this data and causes reputation damage, so they'd avoid storing anything if they can.