Another problem is the way pensions are set up in the U.S.. Most of them are 401k or similar stock market based investment funds.
You're mixing pensions with 401ks, and they're entirely different things.
Pension managers have specific payouts they need to meet, on particular schedules. They might well have reasons to browbeat corporations into doing things.
401k managers don't have any of that. The money they manage is owned by the individuals, there are no specific payouts planned or required, and even how the money is invested is generally directed by the individuals. Of course, most individuals put their money in mutual funds or ETFs, from among the offerings provided by the brokerage. The managers of those funds may have some incentive to manipulate boards to goose their short term returns, but not much because they typically include a broad basket of securities and long-term funds need to have good long-term performance. They could goose the short term returns from a company and then cash out of that position before the damage shows up, but if they do that very much then companies will have strong reasons not to cave into their demands since doing so will hurt the company in both the medium term (when the fund cashes out) and in the long term (when the chickens come home to roost).
I don't think 401ks have anything to do with this sort of short-sightedness. Executive bonuses are a much more plausible explanation.