The acquiring company controls the offer and structures it to maximize their benefit. The acquiring company does not benefit from paying people who no longer work at the company; they're incentivized to maximize the payout to current employees, in particular to aid retention.

In theory, the board of the acquiree thus has a "yes/no" decision to make, and whether or not it screws non-employee common stockholders, the "yes" decision might be in the greatest interest of the maximum number of shareholders.

It's a lot more complicated than this, but if you want an illustration of why it will cost you 6 figures to lose a court case over this issue, there you go.