Few different alternative reasons: Capital gains are generally taxed at a lower rate than bonuses/salary - therefore in after-tax cash terms a $500,000 signing bonus is not the same as selling shares for $500,000. Potentially different tax treatment from the acquirer side as well.
Also - Investors are commonly the ones that orchestrate the talent acquisitions - therefore they want to get their share of the pie.
Also - shareholders of public companies perceive acquisitions differently than large salaries, both in technical terms (i.e. where the transaction appears on financial statements) as well as psychologically speaking.
Also - its potentially more complex to negotiate individual hires, especially if its a large team. (i.e. Some employees may choose to walk away from an individual hire - maybe the employees you really want. They don't really have that luxury if the deal is dependent on all-or-nothing)
Also - Investors are commonly the ones that orchestrate the talent acquisitions - therefore they want to get their share of the pie.
Also - shareholders of public companies perceive acquisitions differently than large salaries, both in technical terms (i.e. where the transaction appears on financial statements) as well as psychologically speaking.
Also - its potentially more complex to negotiate individual hires, especially if its a large team. (i.e. Some employees may choose to walk away from an individual hire - maybe the employees you really want. They don't really have that luxury if the deal is dependent on all-or-nothing)