Suppose you built a successful small company but, suddenly, every employee quit at once. Could your business carry on the next day? Could it survive until you hired and trained replacements?
If the answer is "no" then you have made the case that employees both deserve to share in the business's success and will probably be incentivised by co-owning the company.
Does the janitor deserve as much of the profits as the CTO? Well, what premium do you put on your other employees not getting sick, or injuring themselves?
(Wasn't there a case where a Google chef made a fortune from stocks? Much to the chagrin of some?)
The opposite argument is that those who risk capital are the only ones who deserve the reward. Without investment, a company can't launch or grow. Workers are an operational cost - they are paid for labour and no more deserving of reward than the electricity company. Both provide a service but neither takes a risk.