Because you are talking about "shares" not money in funds where most
Americans have their retirement money. It doesn't really matter
whether that is a 401k/IRA, a private pension plan or a
union/government employee plan. A significant part of all of that
money is in equities and it is ultimately the worker's (later
retiree's) money.
The only exception I can think of is the federal government and they
have no real investments at all other than whatever our kids can bear
in taxes.
What percentage of equities, directly or indirectly through funds, are
held by the 10% of the wealthiest?