This touches on the core of how the founders are motivating their workers. From your point of view, it isn't working and it sounds like this view is shared by others. There are a couple of ways to bring this up to the founders, though it does depend on their personalities; if they're fundamentally not approachable about your only option is unpleasant direct-action (and most people would rather get another job than bother).
The Direct Approach: One on One
If your founders are the kind who encourage an open environment, break down barriers between decision makers and product makers, and otherwise state they're always willing to accept feedback, take them up on their word. Be the bold one, and actually schedule a chat with them.
All these soft awards are nice and all, but we're tired. Maybe we need some hard awards, with money behind them.
Since they're worried about the investors, pitch with them in mind.
Since the investors don't want us throwing money around just for fun, why not do it targeted? Set up some rewards with money behind them, for goals we reach. That's not celebration, that motivation. Which we need.
The Direct Approach: Many to One
If your peers really feel the same way you do, and your founders are kind of aloof (the previous approach won't work), it's time to come as a group. You'd make the same argument you would in the one-on-one approach, but you add more people behind it.
Maybe they need to hear the same thing from multiple people (iterative one-on-one).
Maybe they need to hear the same thing from a group at the same time.
The second method is big guns, and really should only be done if the corporate culture requires it ("feedback is done where everyone can see it") or the founders really do need a knock that hard to notice the problem.
The Indirect Approach
If they really aren't approachable, and you can't get backing for a group to present their grievances, you may need to get subtle.
So they want to take you all out for a weekend. Oh shucks, most of you have conflicts or can't afford it, sorry.
Once the workers have spurned a 'reward' like that, in such quantity, they may be better prepared for one of the previous approaches. They want to know why everyone suddenly doesn't care, and are primed to find out why. So you tell them.
The key arguments to make are in the one-on-one option: you need to speak their language (small business owner) and demonstrate a way out.