Employees might want to receive the most cash they can right now (and therefore prefer 100 units of cash over 90 units of cash and 10 of equity compensation). We shouldn't force them to accept a different mix of compensation, particularly one which forces them to invest in their employer via such a tradeoff. They might choose to work at business X instead of worker-owned cooperative Y for any number of reasons.
Employers think about the value proposition they offer to attract and retain employees and if there are 100 units of compensation available to be paid and they have reason to think that employees prefer cash over equity, they are likely to offer all 100 units of comp in cash rather than 90 units of cash and 10 units of equity. There's no sense tying up 10 units of equity that an employee only values as being equivalent to 5 units of cash.
In the case of workers working for public companies, they have a straightforward way to invest in their company if they want: take some of the cash and buy shares in the company. That's barred in most cases for private companies by practicality and accredited investor rules.