What about the one person who actually can't afford to do that because they'd already budgeted assuming that money was coming in?
What about if you need to reduce headcount? Do you just all vote on who should be fired in effectively a popularity contest that will cause build up of resentment later on? Draw the shortest straw? In this case, an "owner" is probably best because they can all collectively "hate" the owner and bond together as staff.
What about if someone is secretly planning to leave in the next few months and wants their money now as salary rather than investing in long term projects for the company, etc... They might destabilise the long-term success of the company if they vote not to invest, but be unmotivated and work poorly if they know some of "their money" was invested and they're never going to see it.
There seem to be so many edge cases where the traditional hierarchy structure seems like the best viable solution. There are some examples of co-operatives in the UK, the most famous is John Lewis. I think they work around it by just running mostly like a normal business, paying fairly normal salaries for different roles, but then bonuses for profit sharing.