It's the easiest thing for employers to do. I've seen exceptionally qualified candidates in the back half of their 50's who get laid off struggle to find lateral roles and even gigs that are a slight step down. "Overqualified" is a perfectly acceptable legal basis to reject applicants. Quite often, the real reason is that they expect these folks to give them not a minute past their 62nd birthday. If the candidate is 60, companies often don't want to be backfilling the role again in 2 years.
Taking away that safety net is never going to be popular with voters over 45/50.
One of the "fair share" ethos that I fully support is removing the tax cap on Social Security withholdings. It's quite literally a regressive income tax rate to do it that way.